title: Determination of the membrane topology of the small EF-hand Ca2+-sensing proteins CaBP7 and CaBP8.
abstract: The CaBPs represent a subfamily of small EF-hand containing calcium (Ca(2+))-sensing proteins related to calmodulin that regulate key ion channels in the mammalian nervous system. In a recent bioinformatic analyses we determined that CaBP7 and CaBP8 form an evolutionarily distinct branch within the CaBPs (also known as the calneurons) a finding that is consistent with earlier observations characterising a putative C-terminal transmembrane (TM) spanning helix in each of these proteins which is essential for their sub-cellular targeting to the Golgi apparatus and constitutive secretory vesicles. The C-terminal position of the predicted TM-helix suggests that CaBP7 and CaBP8 could be processed in a manner analogous to tail-anchored integral membrane proteins which exhibit the ability to insert across membranes post-translationally. In this study we have investigated the topology of CaBP7 and CaBP8 within cellular membranes through a combination of trypsin protection and epitope accessibility analyses. Our results indicate that the TM-helices of CaBP7 and CaBP8 insert fully across membranes such that their extreme C-termini are luminal. The observed type-II membrane topology is consistent with processing of CaBP7 and CaBP8 as true tail-anchored proteins. This targeting mechanism is distinct from any other calmodulin related Ca(2+)-sensor and conceivably underpins unique physiological functions of these proteins.
authors: Department of Cellular and Molecular Physiology, University of Liverpool, Liverpool, United Kingdom.
authors: Hannah V
authors: HV
authors: McCue
authors: Robert D
authors: RD
authors: Burgoyne
authors: Lee P
authors: LP
authors: Haynes
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D002147:Calmodulin / Q000737:chemistry / Q000378:metabolism*; D017931:DNA Primers; D006367:HeLa Cells; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D013347:Subcellular Fractions / Q000378:metabolism
chemical_list: C540140:CABP7 protein, human; C428830:CALN1 protein, human; D002135:Calcium-Binding Proteins; D002147:Calmodulin; D017931:DNA Primers; D008565:Membrane Proteins