US2, a human cytomegalovirus-encoded type I membrane protein, contains a non-cleavable amino-terminal signal peptide.

The human cytomegalovirus US2 gene product targets major histocompatibility class I molecules for degradation in a proteasome-dependent fashion. Degradation requires interaction between the endoplasmic reticulum (ER) lumenal domains of US2 and class I. While ER insertion of US2 is essential for US2 function, US2 lacks a cleavable signal peptide. Radiosequence analysis of glycosylated US2 confirms the presence of the NH(2) terminus predicted on the basis of the amino acid sequence, with no evidence for processing by signal peptidase. Despite the absence of cleavage, the US2 NH(2)-terminal segment constitutes its signal peptide and is sufficient to drive ER translocation of chimeric reporter proteins, again without further cleavage. The putative US2 signal peptide c-region is responsible for the absence of cleavage, despite the presence of a suitable -3,-1 amino acid motif for signal peptidase recognition. In addition, the US2 signal peptide affects the early processing events of the nascent polypeptide, altering the efficiency of ER insertion and subsequent N-linked glycosylation. To our knowledge, US2 is the first example of a membrane protein that does not contain a cleavable signal peptide, yet otherwise behaves like a type I membrane glycoprotein.