Evolutionarily Conserved Multiple C2 Domain Proteins with Two Transmembrane Regions (MCTPs) and Unusual Ca2+ Binding Properties

摘要: C2 domains are primarily found in signal transduction proteins such as protein kinase C, which generally contain a single domain, and membrane trafficking synaptotagmins, multiple domains. In both classes of proteins, usually regulate the respective protein's function by forming Ca(2+)-dependent or Ca(2+)-independent phospholipid complexes. We now describe MCTPs (multiple domain transmembrane region proteins), novel family evolutionarily conserved with unusual properties. composed variable N-terminal sequence, three domains, two regions, short C-terminal sequence. The invertebrate organisms Caenorhabditis elegans Drosophila melanogaster express MCTP gene, whereas vertebrates genes (MCTP1 MCTP2) whose primary transcripts extensively alternatively spliced. Most sequences, particular highly conserved. All except for second MCTP2 include perfect Ca2+/phospholipid-binding consensus To determine whether actually modules, we analyzed their Ca2+ binding Surprisingly, that none MCTP1 interacted negatively charged neutral phospholipids presence absence Ca2+. However, titrations monitored via intrinsic tryptophan fluorescence revealed all bound high apparent affinity (EC50 approximately 1.3-2.3 microM). Our data thus reveal anchored closely spaced regions represent Ca(2+)-binding but not phospholipid-binding modules.