A role for Lte1p (a low temperature essential protein involved in mitosis) in proprotein processing in the yeast secretory pathway.
We previously identified six single gene disruptions in Saccharomyces cerevisiae that allow enhanced immunoreactive insulin secretion primarily because of defective Kex2p-mediated endoproteolytic processing. Five eis mutants disrupted established VPS (vacuolar protein sorting) genes, The sixth, LTE1, is a Low Temperature (<15 degrees C) Essential gene encoding a large protein with potential guanine nucleotide exchange (GEF) domains. Lte1p functions as a positive regulator of the mitotic GTPase Tem1p, and overexpression of Tem1p suppresses the low temperature mitotic defect of lte1. By sequence analysis, Tem1p has highest similarity to Vps21p (yeast homolog of mammalian Rab5). Unlike TEM1, LTE1 is not restricted to mitosis but is expressed throughout the cell cycle. Lte1p function in interphase cells is largely unknown. Here we confirm the eis phenotype of lte1 mutant cells and demonstrate a defect in proalpha factor processing that is rescued by expression of full-length Lte1p but not a C-terminally truncated Lte1p lacking its GEF homology domain. Neither overexpression of Tem1p nor 13 other structurally related GTPases can suppress the secretory proprotein processing defect. However, overexpression of Vps21p selectively restores proprotein processing in a manner dependent upon the active GTP-bound form of the GTPase. By contrast, a vps21 mutant produces a synthetic defect with lte1 in proprotein processing, as well as a synthetic growth defect. Together, the data underscore a link between the mitotic regulator, Lte1p, and protein processing and trafficking in the secretory/endosomal system.
Zhao
Xiang
X
Division of Metabolism, Endocrinology & Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48109, USA.
Chang
Amy Y
AY
Toh-E
Akio
A
Arvan
Peter
P
D002453:Cell Cycle; D011992:Endosomes / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000502:physiology*; D007328:Insulin / Q000378:metabolism; D008938:Mitosis; D020559:Monomeric GTP-Binding Proteins / Q000378:metabolism; D010641:Phenotype; D010957:Plasmids / Q000378:metabolism; D043484:Proprotein Convertases / Q000378:metabolism; D017434:Protein Structure, Tertiary; D051964:Receptors, Mating Factor / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism / Q000502:physiology*; D013696:Temperature
D005656:Fungal Proteins; D020662:Guanine Nucleotide Exchange Factors; D007328:Insulin; C089357:LTE1 protein, S cerevisiae; D051964:Receptors, Mating Factor; C081863:STE3 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C089965:TEM1 protein, S cerevisiae; D043484:Proprotein Convertases; C058751:KEX2 protein, S cerevisiae; D020558:GTP Phosphohydrolases; D020559:Monomeric GTP-Binding Proteins