Receptor binding specificities of Herstatin and its intron 8-encoded domain.

Retention of intron 8 in alternative HER-2 mRNA generates an inhibitory secreted ligand, Herstatin, with a novel receptor-binding domain (RBD) encoded by the intron. This study examines binding interactions with several receptors and investigates sequence variations in intron 8. The RBD, expressed as a peptide, binds at nM concentrations to HER-2, the EGFR, DeltaEGFR, HER-4 and to the IGF-1 receptor, but not to HER-3 nor to the FGF-3 receptor, whereas a rare mutation in the RBD (Arg to Ile) eliminates receptor binding. The full-length Herstatin binds with 3-4-fold higher affinity than its RBD, but with approximately 10-fold lower affinity to the IGF-IR. Sequence conservation in rhesus monkey but not in rat suggests that intron 8 recently diverged as a receptor-binding module critical for the function of Herstatin.

The role of BiP in endoplasmic reticulum-associated degradation of major histocompatibility complex class I heavy chain induced by cytomegalovirus proteins.

Human cytomegalovirus (HCMV1) US11 and US2 proteins cause rapid degradation of major histocompatibility complex (MHC) molecules, apparently by ligating cellular endoplasmic reticulum (ER)-associated degradation machinery. Here, we show that US11 and US2 bind the ER chaperone BiP. Four related HCMV proteins, US3, US7, US9, and US10, which do not promote degradation of MHC proteins, did not bind BiP. Silencing BiP reduced US11- and US2-mediated degradation of MHC class I heavy chain (HC) without altering the synthesis or translocation of HC into the ER or the stability of HC in the absence of US11 or US2. Induction of the unfolded protein response (UPR) did not affect US11-mediated HC degradation and could not explain the stabilization of HC when BiP was silenced. Unlike in yeast, BiP did not act by maintaining substrates in a retrotranslocation-competent form. Our studies go beyond previous observations in mammalian cells correlating BiP release with degradation, demonstrating that BiP is functionally required for US2- and US11-mediated HC degradation. Further, US2 and US11 bound BiP even when HC was absent and degradation of US2 depended on HC. These data were consistent with a model in which US2 and US11 bridge HC onto BiP promoting interactions with other ER-associated degradation proteins.