The Role of pUL138 In HCMV Persistence

Human cytomegalovirus (HCMV) coexists indefinitely in infected individuals through a poorly characterized latent infection in hematopoietic cells. We previously demonstrated a requirement for UL138 in promoting a latent infection in CD34+ hematopoietic progenitor cells (HPCs). UL138 is encoded on three co-terminal transcripts of, 1.7-, 2.7-, and 3.6-kilobases. Interestingly, the UL138 protein product (pUL138) is necessary but insufficient for HCMV latency. The mechanisms by which pUL138 contributes to the latent infection are unknown, however other viral determinants are required for the latent infection. We identified 3 novel proteins pUL133, pUL135, and pUL136 encoded on the UL138 transcripts. Similar to pUL138, pUL133, pUL135, and pUL136 are Golgi localized type I transmembrane proteins expressed with early kinetics during productive infection. We have named these UL138 related proteins, CLAMPs for HCMV Latency Associated Membrane Proteins. Through a systematic immunoprecipitation analysis, we identified interactions between the CLAMPs and characterized an interaction between pUL133 and pUL138. Further, we mapped the interacting region to a specific domain in the C-terminal, cytosolic tail of pUL138. Additionally, we show that each of the CLAMPs has the ability to self-associate. The localization of the CLAMPs to the Golgi suggests that these proteins likely promote HCMV latency through a novel mechanism involving Golgi functions. Additionally, through a Y2H screen of a human bone marrow cDNA library, we identified an interaction between pUL138 and the heat shock protein 40 (Hsp40) variant MRJ. We confirmed this interaction in mammalian cells and mapped the pUL138 region responsible for this interaction to a domain in the cytoplasmic tail of pUL138. We also demonstrated additional MRJ interactions with pUL133 and pUL136. Importantly, pUL138 specifically interacts with Hsp40 variants during productive infection. Preliminary data suggest that HCMV infection up regulates MRJ mRNA expression and recombinant viruses lacking pUL138 show a disproportionate up regulation of MRJ. pUL138 is the first HCMV protein demonstrated to promote a latent infection. While the mechanisms by which pUL138 contributes to latency remain unknown, the interaction with other CLAMPs and with MRJ, suggest that pUL138 may cooperate with other CLAMPs to modulate the cellular stress response at the Golgi to promote HCMV latency