The E1B 19K protein associates with lamins in vivo and its proper localization is required for inhibition of apoptosis

Abstract

Expression of the E1B 19K protein is required to inhibit apoptosis induced by E1A during adenovirus infection and transformation. E1B 19K is homologous to Bcl-2 in function and the two proteins also share limited amino acid sequence homology. Consequently, the E1B 19K and Bcl-2 proteins bind to and inhibit the cellular deathinducing proteins Bax, Bak and Nbk/Bik. Both E1B 19K and Bcl-2 localize to membranes of the nucleus and the endoplasmic reticulum. In addition to membrane association, and unlike Bcl-2, the E1B 19K protein is found associated with intermediate ®lament proteins in the cytoplasm and the nuclear lamina and copuri®es with the lamins both during infection and transformation. While a membrane targeting domain at the C-terminus of Bcl-2 ensures its proper localization, the mechanism by which the E1B 19K protein localizes is unknown. Not surprisingly, lamin A fragments were cloned from a yeast two-hybrid screen for E1B 19K-interacting proteins. The interaction was demonstrated in yeast and mammalian cells in vivo and in vitro and was unique and speci®c to E1B 19K, with no interaction evident between Bcl-2 and lamin A. Mutants of lamin A/C which localized inappropriately in the cytoplasm or nucleus but retained E1B 19K binding, interfered with the nuclear envelope and cytoplasmic membrane targeting of the E1B 19K protein. Improper localization impaired the ability of the E1B 19K protein to inhibit apoptosis. Thus, proper localization of the E1B 19K protein is required for its function and the interaction of the E1B 19K protein with lamin A/C may represent a means for nuclear envelope localization.

Mechanisms of apoptosis regulation by viral oncogenes in infection and tumorigenesis

“…We identified BAX, 24 a protein that had been identified as a proapoptotic BCL-2 interacting protein in the Korsmeyer laboratory 25 ; natural born killer/BCL-2-interacting killer/BIKlike killer (NBK/BIK/BLK), a proapoptotic protein and the first BH3-only protein to be identified 26 ; and lamin A, which is involved in localization of E1B 19K to the nuclear envelope. 27 Other groups identified proapoptotic BAK as an E1B 19K and BCL-2 interacting protein [28][29][30] and there are a number of other joint E1B 19K/BCL-2 interacting proteins that remain poorly characterized. 31 These and other findings revealed that the BCL-2 family of proteins consists of proapoptotic and antiapoptotic members that are regulated by functional antagonism through protein-protein interactions.…”

Transient expression of the Bcl-2 family member, A1-a, results in nuclear localization and resistance to staurosporine-induced apoptosis

“…Mutations that cause mislocalization of the lamins, and thus change the localization of E1B 19K protein, lead to abrogation of the E1B 19K anti-apoptotic activity. 41 A possible mechanism E1B 19K may use to block apoptosis may involve sequestration of pro-apoptotic proteins. The E1B 19K protein has been shown to sequester CED-4, the C. elegans APAF-1 homologue, from the cytosol to the nucleus, abrogating the ability of CED-4 to activate caspase-8.…”

Subcellular and submitochondrial mode of action of Bcl-2-like oncoproteins

“…Thus, Bcl-2, Bcl-X L , Bax, and the Epstein ± Barr virus gene product BHRF tend to be particularly abundant in the outer mitochondrial membrane (Cory, 1995;Gonzalez-Garcia et al, 1994;Krajewski et al, 1993;Reed, 1994;Riparbelli et al, 1995;Yang and Korsmeyer, 1996;Zha et al, 1996a). In contrast, E1B19K, which lacks a TM domain, is mostly localized in the nuclear envelope (Rao et al, 1997). It appears unlikely that mere TM-lipid interactions determine the subcellular localization of these proteins.…”

“…It appears unlikely that mere TM-lipid interactions determine the subcellular localization of these proteins. The preferential localization of E1B19K may be dictated by protein-protein interactions with nuclear lamins A/C (Rao et al, 1997), whereas, in the case of Bcl-2, interactions with mitochondrial outer membrane proteins (perhaps carnitine palmitoyltransferase I?) (Paumen et al, 1997) may be important.…”

“…However, Bcl-2 can protect cytoplasts (that is anucleate cells) against apoptosis Decaudin et al, 1997;Jacobson et al, 1994). In the case of E1B19K, nevertheless, evidence is available suggesting that this protein loses its anti-apoptotic e ect when it is targeted to an extranuclear localization by overexpressing mutants of lamin A/C which localize inappropriately to the cytoplasm (Rao et al, 1997). In summary, the nuclear localization of Bcl-2-related proteins can have some impact on speci®c (private) pathways of apoptotic signaling, yet is unlikely to account for the broad cytoprotective e ect of Bcl-2.…”