C-Terminal Truncation of α-COP Affects Functioning of Secretory Organelles and Calcium Homeostasis in Hansenula polymorpha

Abstract

ABSTRACT In eukaryotic cells, COPI vesicles retrieve resident proteins to the endoplasmic reticulum and mediate intra-Golgi transport. Here, we studied the Hansenula polymorpha homologue of the Saccharomyces cerevisiae RET1 gene, encoding α-COP, a subunit of the COPI protein complex. H. polymorpha ret1 mutants, which expressed truncated α-COP lacking more than 300 C-terminal amino acids, manifested an enhanced ability to secrete human urokinase-type plasminogen activator (uPA) and an inability to grow with a shortage of Ca 2+ ions, whereas a lack of α-COP expression was lethal. The α-COP defect also caused alteration of intracellular transport of the glycosylphosphatidylinositol-anchored protein Gas1p, secretion of abnormal uPA forms, and reductions in the levels of Pmr1p, a Golgi Ca 2+ -ATPase. Overexpression of Pmr1p suppressed some ret1 mutant phenotypes, namely, Ca 2+ dependence and enhanced uPA secretion. The role of COPI-dependent vesicular transport in cellular Ca 2+ homeostasis is discussed.