Alternative translation initiation gives rise to two isoforms of Orai1 with distinct plasma membrane mobilities.
Store-operated calcium entry is an almost ubiquitous signaling pathway in eukaryotic cells. The plasma membrane store-operated channels are comprised of subunits of the recently discovered Orai proteins, the major one being Orai1.We have discovered that native Orai1, as well as expressed Orai1, exists in two forms in similar quantities: a longer form (Orai1α) of approximately 33 kDa, and a shorter form (Orai1β) of approximately 23 kDa. The Orai1β form arises from alternative translation initiation from a methionine at position 64, and possibly also 71, in the longer Orai1α form. In the sequence upstream of the initiation site of Orai1β, there is a poly-arginine sequence previously suggested to be involved in interaction of Orai1 with plasma membrane phosphatidylinositol-4,5-bisphosphate. The loss of this phospholipid binding domain would be expected to influence the mobility of Orai1 protein in the plasma membrane. Indeed, experiments utilizing fluorescence recovery after photobleaching (FRAP) revealed that the recovery half-time for Orai1β was significantly faster than for Orai1α. Since Orai1 must diffuse to sites of interaction with the Ca(2+) sensor, STIM1, these two mobilities might provide for efficient recruitment of Orai1 subunits to sites of store-operated Ca(2+) entry during agonist-induced Ca(2+) signaling.
National Institute of Environmental Health Sciences-NIH, Department of Health and Human Services, Research Triangle Park, NC 27709, USA.
Miwako
M
Fukushima
Takuro
T
Tomita
Agnes
A
Janoshazi
James W
JW
Putney
D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000737:chemistry / Q000378:metabolism*; D020013:Calcium Signaling; D002462:Cell Membrane / Q000378:metabolism*; D036681:Fluorescence Recovery After Photobleaching; D057809:HEK293 Cells; D006801:Humans; D000071740:ORAI1 Protein; D010442:Peptide Chain Initiation, Translational; D020033:Protein Isoforms / Q000737:chemistry / Q000378:metabolism; D021381:Protein Transport; D013347:Subcellular Fractions / Q000378:metabolism
D015220:Calcium Channels; D000071740:ORAI1 Protein; C510746:ORAI1 protein, human; D020033:Protein Isoforms; D002118:Calcium