Cross-Reference to Related Applications This application claims the benefit of priority of U.S. Provisional Application ,-No. 60/400410, filed August 1, 2002, the specification of which is incorporated by reference herein hi its entirety. Background Tumor necrosis factor a (TNF-a) is a pro-inflammatory cytokine produced primarily by macrophages. The pleiotropic actions of TNF-« include inflammation, cell proliferation, differentiation, and apoptosis (Tracey et al, (1993) Annu. Rev. Cell Biol. 9:317-313; Baud et al., (2001) Trends in Cell Biol. 11:372-377). These actions are initiated by the binding of TNF-a to its receptors (i.e., TNFRs) that are expressed on most kinds of cells (Bag!ioni etal,, 1985; Beutleret al., 1985; Kult etal., I9S5: Tsujirooto et al., 1985; Aggarwal et al., 1985; Israel el al., 1986). The receptors provide the intrae«ll«lar signals for cell response to TNF-a (Engelmami et aL 1990a). TNF-a and TNFR play a role in inflammatory response. On one hand, TNF-a stimulates immunity, conferring resistance to infectious agents and resistance to tumors (Vilcek, et al., (1991) J. Biol. Chem. 266:7313-7316). On the olher hand, TNF-a is implicated in a number of autoimmune diseases such as rheumatoid arthritis, graft rejection, and graft-versus-host diseases (Beutler, etal., (1998) Blood Cells Mol. Dis. 24:216-230; Beutler, (1999) J. Rheumatol. 26(Swppl) 57:16-21). TNF-a and TNFR play another role in apoptosis, or programmed cell death. Apoptosis is a physiologic process essential to the normal development and homeostasis of multiceilular organisms (H. Steller. (1995) Science 267:1445-1449). Derangements of apoptosis contribute to the pathogenesis of several human diseases including cancer, neurodegenerative disorders, and acquired immune deficiency syndrome (C.B. Thompson, (!995) Science 267:1456-1462). The effects of TNF-a figands and receptors are varied and influence numerous functions, both normal and abnormal, in the biological processes of the mammalian sy.stem. There is a cleat need, therefore, for identification and characterization of protein complexes comprising such receptors and ligands, which influence biological activity, both normally and in disease states. Brief Summary In certain aspects, the invention provides an isolated, purified, or recombinant protein complex comprising a TNF-a polypeptide, a TNFR polypeptide and at least one polypeptide selected from the group consisting of: NF-KB activating kinase (NAK),-RasGAPS, TRCP1, and TRCP2. NAK is also known as TBK1 (TANK-binding kinase) or T2K (TRAF2-associated kinase). In certain embodiments, the isolated, purified, or recombinant protein complex further comprises at least one polypeptide selected from the group consisting of: TRADD, TRAF2, and TRAP2. In certain aspects, the invention provides an isolated, purified, or recombinant protein complex comprising a TNFR polypeptide and at least one p^l>^rjeptide_selected from the group consisting of: NF-KB activating kinase (NAK), RasGAPS, TRCP1, and TRCP2. In certain embodiments, the isolated, purified, or recombinant protein complex furtljer comprises at least one polypeptide selected from the group consisting of: TNF-a, TRADD, TRAF2, and TRAP2. In a specific embodiment, the protein complex of the present invention comprises a TNF-a polypeptide, a TNFR polypeptide, a NAK polypeptide, a RasGAPS polypeptide, a TRCP1 polypeptide, a TRCP2 polypeptide. a TRADD polypeptide, a TRAF2 polypeptide, and a TRAP2 polypeptide. For example, the TNFR polypeptide of the complex can be a TNFR1 or TNFR2 polypeptide. In certain embodiments, one or more of the polypeptides of a complex of the invention is a variant, such as a fragment, a fusion protein, a labeled protein, etc., and preferably the variant is a functional variant. In further aspects, the invention provides host cells comprising one or more recombinant nucleic acids encoding one or more polypeptide constituents of a complex disclosed herein. In certain embodiments, the host cells comprise a first nucleic acid, a second nucleic acid and a third nucleic acid, wherein the first nucleic acid comprises a recombinant nucleic acid encoding a TNFR polypeptide, the second nucleic acid comprises a recombinant nucleic acid encoding a TNF-a polypeptide. and the third nucleic acid comprises a recombinant nucleic acid encoding a polypeptide selected from the group consisting of: NAK, RasGAPS, TRCP1, and TRCP2. In certain embodiments, the host cells comprise a first nucleic acid and a second nucleic acid, wherein the first nucleic acid comprises a. recombinant nucleic acid encoding a TNFR polypeptide, and wherein the second nucleic acid comprises a recombinant nucle'ic acid encoding a pohpeptide selected from the group consisting of: NAK. RasGAPS, TRCP1, and TRCP2. In certain aspects, the invention provides assays for identifying test compounds that inhibit or potentiate the stability of a protein complex disclosed herein. In certain embodiments, an assay comprises: forming a reaction mixture including TNF-€ell 72:223-232; Madura etal., (1993) J. Biol. Chem. 268:12046-12054;_Barte|jBt_gl.r(i993) Biotechniques 14:920-924; and Iwabuchi et al,. (1993) Oncogene 8:1693-1696), and for subsequently detecting test compounds which inhibit or potentiate binding of the proteins to one and other. This assay includes providing a host cell, for example, a yeast cell (preferred), a mammalian cell or a bacterial cell type. The host cell contains a reporter gene having a binding site for the DNA-binding domain of a transcriptional activator used in the bait protein, such that the reporter gene expresses a detectable gene product when the gene is transcriptionaliy activated. A first chimeric gene is provided which is capable of being expressed in the host cell, and encodes a "bait" fusion protein. A second chimeric gene is also provided which is capable of being expressed in the host cell, and encodes the "fish" fusion protein. In one embodiment, both the first and the second chimeric genes are introduced into the host cell in the form of plasmids. Preferably, however, the first chimeric gene is present in a chromosome of the host cell and tire second chimeric gene is introduced into the host cell as part of a plasmid. Preferably, the DNA-binding domain of the first hybrid protein and the transcriptional activation domain of the second hybrid protein are derived from transcriptional activators having separable DNA-binding and transcriptional activation domains. For instance, there separate DNA-binding and transcriptional activation domains are known to be found in the yeast GAL4 protein, and are known to be found in the yeast GCN4 and ADR1 proteins. Many other proteins involved in transcription also have separable binding and transcriptional activation domains which make them useful for the present invention, and include, for example, the LexA and VP16 proteins. It will be understood that other (substantially) transcriptionally-inert DNA-binding ' domains may be used in the subject constructs; such as domains of ACE1, lei, lac represser, jun or fos. In another embodiment, the DNA-binding domain and the transcriptional activation domain may be from different proteins. The use of a LexA DNA binding domain provides certain advantages. For example, in yeast, the LexA moiety contains no activation function and has no known effect on transcription of yeast genes. In addition, use of LexA allows control over the sensitivity of the assay to the level of interaction (see, for example, the Brent et al., PCT publication WO94/10300). In certain embodiments, the invention provides a two-hybrid assay to identify test compounds that inhibit or potentiate the stability of the complex. To illustrate, a first fusion protein (i.e., a "bait" protein) comprising a TNFR polypeptide and a second fusion protein (i.e., a "fish" protein) comprising a polypeptide selected from the group consisting of: NAK, RasGAP3, TRCP1, and TRCP2, are introduced in the host cell. Cells are subjected to conditions under which the bait and fish fusion proteins are expressed in sufficient quantity for the reporter gene to be activated. The interaction of the two fusion polypeptides of the complex results in a detectable signal produced by the expression of the reporter gene. Accordingly, the level of interaction between the two fusion proteins in the presence of a test compound and in the absence of the test compound can be evaluated by detecting the level of expression of the reporter gene in each case. Various reporter constructs may be used in accord with the methods of the invention and include, for example, reporter genes which produce such detectable signals as selected from the group consisting of: an enzymatic signal, a fluorescent signal, a phosphorescent signal and drug resistance. In many drug screening programs which test libraries of compounds and natural extracts, high throughput assays are desirable in order to maximize the number of compounds surveyed in a given period of time. Assays of the present invention which are performed in cell-free systems, such as may be developed with purified or semi-purified proteins or with lysates, are often preferred as "primary" screens in that they can be generated to permit rapid development and relatively easy detection of an alteration in a molecular target which is mediated by a test compound. Moreover, the effects of cellular toxicity and/or bioavailability of the test compound can be generally ignored in the in vitro system, the assay instead being focused primarily on the effect of the drug on the molecular target as may be manifest in an alteration of binding affinity with other proteins or changes in enzymatic properties of the molecular target. In certain embodiments, activities of a protein complex may include, without limitation, a protein complex formation, which may be assessed by immunoprecipitation and analysis of co-immunoprecipitated proteins or affinity purification and analysis of co-purified proteins. Fluorescence Resonance Energy Transfer (FRET)-based assays may also be used to determine complex formation. Fluorescent molecules having the proper emission and excitation spectra that are brought into close proximity with one another can exhibit FRET. The fluorescent molecules are chosen such that the emission spectrum of one of the molecules (the donor molecule) overlaps with the excitation spectrum of the other molecule (the acceptor molecule). The donor molecule is excited by light of appropriate intensity within the donor's excitation spectrum. The donor then emits the absorbed energy as fluorescent light. The fluorescent energy it produces is quenched by the acceptor molecule. FRET can be manifested as a reduction in the intensity of the fluorescent signal from the donor, reduction in the lifetime of its excited state, and/or re-emission of fluorescent light at the longer wavelengths (lower energies) characteristic of the acceptor. When the fluorescent proteins physically separate, FRET effects are diminished or eliminated. (