Multilayered mechanism of CD4 downregulation by HIV-1 Vpu involving distinct ER retention and ERAD targeting steps

JAVIER G. MAGADAN

Mendoza

Workshop; Intracellular pathogens targeting reproductive health and newborns; 2015

IHEM-CONICET, Facultad de Ciencias Médicas, Universidad Nacional de Cuyo

A key function of the Vpu protein of HIV-1 is the targeting of newly-synthesized CD4 for proteasomal degradation. This function is currently thought to require prior ER retention of CD4 by the viral envelope protein, Env, and to occur by a mechanism that is fundamentally distinct from the ER-associated degradation (ERAD) pathway. However, using a combination of genetic, biochemical and morphological methodologies, we find that Vpu-induced degradation is dependent on a key component of the ERAD machinery, the VCP-UFD1L-NPL4 complex, as well as on SCFβ-TrCP-dependent ubiquitination of the CD4 tail on lysine and serine/threonine residues. Remarkably, when degradation is blocked by either inactivation of the complex or prevention of ubiquitination, Vpu still retains the bulk of CD4 in the ER mainly through transmembrane domain interactions. Addition of an ER export signal from the VSV-G protein overrides this retention. Thus, Vpu exerts two distinct activities in the process of downregulating CD4: ER retention followed by targeting to late stages of ERAD. The multiple levels at which Vpu engages these cellular quality control mechanisms underscore the importance of ensuring profound suppression of CD4 to the life cycle of HIV-1