mRNAs Encoding Polarity and Exocytosis Factors Are Cotransported with the Cortical Endoplasmic Reticulum to the Incipient Bud in Saccharomyces cerevisiae

Abstract

Polarized growth in the budding yeast Saccharomyces cerevisiae depends upon the asymmetric localization and enrichment of polarity and secretion factors at the membrane prior to budding. We examined how these factors (i.e., Cdc42, Sec4, and Sro7) reach the bud site and found that their respective mRNAs localize to the tip of the incipient bud prior to nuclear division. Asymmetric mRNA localization depends upon factors that facilitate ASH1 mRNA localization (e.g., the 3 untranslated region, She proteins 1 to 5, Puf6, actin cytoskeleton, and a physical association with She2). mRNA placement precedes protein enrichment and subsequent bud emergence, implying that mRNA localization contributes to polarization. Correspondingly, mRNAs encoding proteins which are not asymmetrically distributed (i.e., Snc1, Mso1, Tub1, Pex3, and Oxa1) are not polarized. Finally, mutations which affect cortical endoplasmic reticulum (ER) entry and anchoring in the bud (myo4⌬, sec3⌬, and srp101) also affect asymmetric mRNA localization. Bud-localized mRNAs, including ASH1, were found to cofractionate with ER microsomes in a She2-and Sec3-dependent manner; thus, asymmetric mRNA transport and cortical ER inheritance are connected processes in yeast.

Localization of mRNAs coding for peroxisomal proteins in the yeast, Saccharomyces cerevisiae

“…In contrast, other peroxin mRNAs, such as PEX6, 10, and 27-29, showed a low level of colocalization with peroxisomes (33%, 30%, 32%, 40%, and 32%, respectively), while some (i.e., PEX2, 7,17,18,22,30, and 32) showed little to no colocalization (6%, 5%, 24%, 18%, 18%, 16%, and 20%, respectively; Fig. 1 A and Table S2).…”

“…The reason for this is unclear, but it could be due to the elevated levels of POT1 mRNA in the cell lysate or because the MS2L sequence interferes with POT1 mRNA localization. Importantly, mRNAs known to localize with cortical ER (i.e., CDC42 and SEC4) (7) were not observed in the peroxisome fraction, although SEC61 mRNA was. This may represent nuclear ER contamination; studies have shown that peroxisomes are derived from ER (19).…”

“…The latter include the She2 RNA-binding protein (RBP) that binds ASH1 mRNA and She1/Myo4, a type V myosin that transports ribonucleoprotein (RNP) particles (5,6). In addition, mRNAs encoding polarity and secretion factors (e.g., Sec4, Sro7, Cdc42) also target to the bud tip to facilitate cell growth (7). These mRNAs use the She machinery as well and, along with ASH1 mRNA, anchor to the endoplasmic reticulum (ER) and are transported to the incipient bud (7,8).…”

“…In addition, mRNAs encoding polarity and secretion factors (e.g., Sec4, Sro7, Cdc42) also target to the bud tip to facilitate cell growth (7). These mRNAs use the She machinery as well and, along with ASH1 mRNA, anchor to the endoplasmic reticulum (ER) and are transported to the incipient bud (7,8). mRNA anchoring to the ER allows for the cotransport of both message and translation/ translocation machinery, and is conserved through evolution (8).…”

Structure of a myosin•adaptor complex and pairing by cargo

“…She3p contains at least three binding modules, one for Myo4p and two for cargos, as diverse as zipcoded mRNP and tubular endoplasmic reticulum (tER) (27)(28)(29). She3p's binding site for Myo4p has been localized to the middle region of She3p (30), with a genetically determined upstream binding site for tER and a downstream binding site for zipcoded mRNP (29).…”