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A Sec7-related protein in Paramecium

Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York 10461-1602, USA

¹Correspondence: Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, 1300 Morris Park Ave., Bronx, New York 10461-1602, USA. E-mail: satir{at}aecom.yu.edu

We have cloned and sequenced a SEC7-related gene in Paramecium tetraurelia that contains an open reading frame for 1135 amino acids encoding a 133 kDa protein, PSec7. Sec7, first identified in vesicular trafficking mutants in yeast, and its phylogenetic homologues function as guanine-nucleotide exchange factors for small G-proteins such as ARF (ADP-ribosylation factor). The deduced amino acid sequence in PSec7 for the motifs that form the ARF binding site are more than 70% identical to yeast Sec7 and similarly identical to ARNO, the human ARF exchange factor, with correct positioning of the critical glutamic acid residue within the motif region. Overall, the identity of PSec7 to yeast Sec7 is 32%. The deduced amino acid sequence also has five sequences that resemble IQ motifs, EF hand binding domains found in all myosins, and two pleckstrin homology domains. Similar sequences are present in yeast Sec7 and other Sec7-related molecules. A protein kinase A phosphorylation site may also be present. Southern blots suggest that a single gene encodes PSec7. Northern blots show that the message encoding PSec7 is induced on deciliation, followed by ciliogenesis, which suggests a role for PSec7 in cilia such as transport or targeting of ciliary membrane components.—Nair, S., Guerra, C., Satir, P. A Sec7-related protein in Paramecium.

Key Words: guanine nucleotide exchange factor • GEF • IQ motifs • cilia

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