C-terminal sequences can inhibit the insertion of membrane proteins into the endoplasmic reticulum of Saccharomyces cerevisiae

Shr3p mediates specific COPII coatomer-cargo interactions required for the packaging of amino acid permeases into ER-derived transport vesicles

The SHR3 gene of Saccharomyces cerevisiae encodes an integral membrane component of the endoplasmic reticulum (ER) with four membrane-spanning segments and a hydrophilic, cytoplasmically oriented carboxyl-terminal domain. Mutations in SHR3 specifically impede the transport of all 18 members of the amino acid permease (aap) gene family away from the ER. Shr3p does not itself exit the ER. Aaps fully integrate into the ER membrane and fold properly independently of Shr3p. Shr3p physically associates with the general aap Gap1p but not Sec61p, Gal2p, or Pma1p in a complex that can be purified from N-dodecylmaltoside-solubilized membranes. Pulse-chase experiments indicate that the Shr3p-Gap1p association is transient, a reflection of the exit of Gap1p from the ER. The ER-derived vesicle COPII coatomer components Sec13p, Sec23p, Sec24p, and Sec31p but not Sar1p bind Shr3p via interactions with its carboxyl-terminal domain. The mutant shr3-23p, a nonfunctional membrane-associated protein, is unable to associate with aaps but retains the capacity to bind COPII components. The overexpression of either Shr3p or shr3-23p partially suppresses the temperature-sensitive sec12-1 allele. These results are consistent with a model in which Shr3p acts as a packaging chaperone that initiates ER-derived transport vesicle formation in the proximity of aaps by facilitating the membrane association and assembly of COPII coatomer components.

Nonlethal sec71-1 and sec72-1 mutations eliminate proteins associated with the Sec63p-BiP complex from S. cerevisiae

The sec71-1 and sec72-1 mutations were identified by a genetic assay that monitored membrane protein integration into the endoplasmic reticulum (ER) membrane of the yeast Saccharomyces cerevisiae. The mutations inhibited integration of various chimeric membrane proteins and translocation of a subset of water soluble proteins. In this paper we show that SEC71 encodes the 31.5-kDa transmembrane glycoprotein (p31.5) and SEC72 encodes the 23-kDa protein (p23) of the Sec63p-BiP complex. SEC71 is therefore identical to SEC66 (HSS1), which was previously shown to encode p31.5. DNA sequence analyses reveal that sec71-1 cells contain a nonsense mutation that removes approximately two-thirds of the cytoplasmic C-terminal domain of p31.5. The sec72-1 mutation shifts the reading frame of the gene encoding p23. Unexpectedly, the sec71-1 mutant lacks p31.5 and p23. Neither mutation is lethal, although sec71-1 cells exhibit a growth defect at 37 degrees C. These results show that p31.5 and p23 are important for the trafficking of a subset of proteins to the ER membrane.

Targeting of passenger protein domains to multiple intracellular membranes

The role of passenger domains in protein targeting was examined by fusing previously characterized targeting motifs to different protein sequences. To compare the targeting requirements for a variety of subcellular compartments, targeting of the fusion proteins was examined for endoplasmic reticulum, mitochondria and peroxisomes in vitro and in yeast. Although most passenger domains were only partially passive to translocation, motif-dependent targeting via motifs positioned at either end of one passenger domain (gPA) was demonstrated for all of the subcellular compartments tested. The data presented extend earlier suggestions that translocation competence is an intrinsic property of the passenger protein. However, the properties that determine protein targeting are not mutually exclusive for the compartments tested. Therefore, although the primary determinant of specificity is the targeting motif, our results suggest that translocation competence of the targeted protein augments the fidelity of transport.

Translocation of proteins across the endoplasmic reticulum

The past year has seen significant advances in the field of protein translocation: the roles of the signal recognition particle and its receptor have been understood in greater detail; many membrane components responsible for translocation have been identified; and insight has been gained into how proteins cross membranes.

Nitrogen catabolite repression of arginase (CAR1) expression in Saccharomyces cerevisiae is derived from regulated inducer exclusion

Expression of the Saccharomyces cerevisiae arginase (CAR1) gene is regulated by induction and nitrogen catabolite repression (NCR). Arginine was demonstrated to be the native inducer. CAR1 sensitivity to NCR has long been accepted to be accomplished through a negative control mechanism, and cis-acting sites for it have been hypothesized. In search of this negatively acting site, we discovered that CAR1 sensitivity to NCR derives from regulated inducer (arginine) exclusion. The route of catabolic entry of arginine into the cell, the general amino acid permease (GAP1), is sensitive to NCR. However, CAR1 expression in the presence of sufficient intracellular arginine is NCR insensitive.