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

Institute of Experimental Cardiology, Cardiology Research Center, 121552 Moscow, Russia

Received 19 July 2003/ Accepted 27 October 2003

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 -COP, a subunit of the COPI protein complex. H. polymorpha ret1 mutants, which expressed truncated -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 Ca²⁺ ions, whereas a lack of -COP expression was lethal. The -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 Ca²⁺-ATPase. Overexpression of Pmr1p suppressed some ret1 mutant phenotypes, namely, Ca²⁺ dependence and enhanced uPA secretion. The role of COPI-dependent vesicular transport in cellular Ca²⁺ homeostasis is discussed.