K15 Protein of Kaposi’s Sarcoma-Associated Herpesvirus Is Latently Expressed and Binds to HAX-1, a Protein with Antiapoptotic Function

Abstract

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the infectious cause of KS and is also linked to the pathogenesis of certain lymphoproliferations (4,14). It is proposed that KSHV latent proteins are directly involved in modulating signal transduction pathways and cellular circuits leading to uncontrolled cell proliferation (2).At the far right-hand end of the KSHV genome, open reading frame (ORF) K15 encodes a putative transmembrane protein in the same genomic location as the Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) (5,7,12,39). K15 resembles LMP2A not only in genomic location but also in its splicing pattern and predicted protein structure. Two highly divergent forms of K15 have been identified: the predominant (P) and minor (M) forms (5, 12, 39). These two alleles possess only 33% amino acid identity yet retain 12 membrane-spanning domains and a putative cytoplasmic signal-transducing carboxyl terminus (C terminus) (5). The C terminus of K15 has potential signaling motifs, including Src homology 2 and 3 binding domains (SH2-B and SH3-B, respectively) (12). A CD8-K15 C-terminal chimeric protein was shown to be constitutively tyrosine phosphorylated within the SH2-B motif (5). Like LMP2A, this CD8-K15 chimeric protein modulates B-cell receptor (BCR) signal transduction. The mechanism(s) of signal transduction is unknown but appears to be distinct from that of LMP2A and does not involve intracellular free calcium mobilization (5).In addition, the C terminus of K15 has sequences similar to those found in EBV LMP1, including a putative tumor necrosis factor receptor-associated factor (TRAF) binding site. K15 therefore appears to be a hybrid of a distant evolutionary relative of both EBV LMP1 and LMP2A (13). The putative C terminus of K15 has been shown to interact with the TRAFs (12), and we have also shown that K15 can indeed activate NF-B via this putative TRAF binding site (unpublished data). By way of activating NF-B, LMP1 of EBV plays an essential role in EBV-induced transformation of B lymphocytes (3,16,21). NF-B activation also appears to be essential for the proliferation potential of KSHV positive primary effusion lymphoma (PEL) cells (22), but whether all of this NF-B activity in PEL cells is due to K15 expression is not yet known.Although K15 mRNA has been demonstrated in PEL cells (5, 12, 39), it is not known whether the K15 protein is actually expressed in latently infected tumor cells. The size of endogenous protein, its exact subcellular localization, and its cellular binding partners have not previously been determined.We generated a monoclonal antibody (MAb) against K15 and show here that when K15 cDNA is ectopically expressed we detect the predicted 50-kDa form as well as a series of smaller proteolytically cleaved forms, of which the 35-and 23-kDa species are predominant. Deletion of the initiator AUG of the K15 ORF abolished protein expression, suggesting that the 50-kDa form of K15 is a precursor which is subsequently proteolytically processed into smaller species. We ...

Omi/HtrA2 is a positive regulator of autophagy that facilitates the degradation of mutant proteins involved in neurodegenerative diseases

“…23 It has also been reported that Hax-1 can localize to the ER. 32 Beclin-1 is located in both the mitochondria and the ER. 22 In this study, Hax-1 co-localized with Beclin-1 (Supplementary Figure S4i).…”

hSav1 interacts with HAX1 and attenuates its anti-apoptotic effects in MCF-7 breast cancer cells

“…HAX1 is known to participate in both death receptor and mitochondria-mediated apoptosis pathways (16) and functions as an endogenous anti-apoptotic molecule. We found that the c-terminal 80-amino acid sequence (208-279) of HAX1, consisting of a transmembrane domain, is required for binding to hSav1.…”

“…HAX1 is involved in both death receptor and mitochondria-mediated apoptosis pathways. Previous studies showed that HAX1 is a potent inhibitor of Bcl-2-associated X protein (Bax)-induced apoptosis (16). Furthermore, it has been demonstrated that HAX1 is degraded in the mitochondria by the high temperature requirement protein A2 (Omi/HtrA2) after induction of apoptosis, which contributes to caspase-independent induction of apoptosis by Omi/HtrA2 (17).…”

HS1-Associated Protein X-1 Interacts with Membrane-Bound IgE: Impact on Receptor-Mediated Internalization

“…Besides its association with HS1, HAX-1 was also reported to physically associate with several other molecules indicating that the biological function of HAX-1 can roughly be divided into three categories: 1) Association of HAX-1 with HS1 (13), Kaposi's sarcomaassociated herpes virus K15 (17), Epstein-Barr nuclear Ag 5 (EBNA5 or EBNA-LP) (18,19), Bcl-2 and BHRF1 (EBV homolog of Bcl-2) (20), Omi/HtrA2, a nuclear-encoded mitochondrial serine protease with proapoptotic function (14) and HIV protein Vpr (21) indicates the involvement of HAX-1 in the regulation of apoptotic processes. 2) Association of HAX-1 with cortactin (or EMS1) (15), polycystic kidney disease protein 2 (15), ATP-binding cassette transporters BSEP, MDR1, and MDR2 (22) and G␣-13, a cell migration-stimulating component (23) emphasizes the involvement of HAX-1 in cell motility processes.…”