Binding protein BiP is required for translocation of secretory proteins into the endoplasmic reticulum in Saccharomyces cerevisiae.

Abstract

The endoplasmic reticulum of mammalian cells contains a heat shock protein of -70 kDa (hsp7O) termed binding protein BiP that is thought to promote the folding and subunit assembly of newly synthesized proteins. To study BiP function, we placed the BiP-encoding gene from Saccharomyces cerevisiae under the control of a regulated promoter and examined the effects of BiP depletion. Reduction of BiP protein to about 15% of normal levels led to a profound reduction in secretion of a factor and invertase. At the same time, unglycosylated precursors of these proteins accumulated intracellularly. The predominant form of the invertase precursor had undergone signal sequence cleavage but accumulated as a soluble species in the cytosol. In contrast, the a-factor precursor was exclusively in the signal-uncleaved form. It sedimented with microsomal membranes and was exposed at the cytoplasmic face in a protease-resistant form. These rmdings suggest that, in yeast, BiP function is required for translocation of soluble proteins into the endoplasmic reticulum at a stage beyond the initial nascent chain-membrane association.

Role of the Mitochondrial DnaJ Homolog Mdj1p as a Chaperone for Mitochondrially Synthesized and Imported Proteins

“…The endoplasmic reticulum lumenal Hsp70, Kar2p (BiP), is essential for mediating posttranslational translocation in S. cerevisiae (35,62). Kar2p interacts with Sec63p, an integral membrane protein possessing a lumenal J domain (4,50).…”

The promoter region of the yeast KAR2 (BiP) gene contains a regulatory domain that responds to the presence of unfolded proteins in the endoplasmic reticulum.

“…9B, top Finally, we examined the effect of overexpression of BiP protein in the ER of yeast cells on the unfolded protein response of the KAR2 promoter. First, we used the glucoserepressible GAL1O promoter, which is 5 to 10 times more active than the wild-type K4R2 promoter in galactosecontaining media but totally inactive in glucose-containing media (42). As shown in Fig.…”

Protein transport in the halophilic archaeon Halobacterium sp. NRC-1: a major role for the twin-arginine translocation pathway?

“…In yeast, in particular, several proteins are translocated posttranslationally. In that case, transport is driven by a pulling action from Kar2p (called BiP in mammalian cells), which is an Hsp70 that resides in the lumen of the ER (Nguyen et al, 1991). Accessory components that are important for protein transport include the Sec62\63 and Sec71\72 complexes, both of which are involved in post-translational translocation (Johnson & Waes, 1999), and TRAM, which is important for the translocation and integration of most secretory proteins (Voigt et al, 1996).…”