Membrane‐bound annexin V isoforms (CaBP33 and CaBP37) and annexin VI in bovine tissues behave like integral membrane proteins

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Abstract
The distribution of annexin V isoforms (CaBP33 and CaBP37) and of annexin VI in bovine lung, heart, and brain subfractions was investigated with special reference to the fractions of these proteins which are membrane‐bound. In addition to EGTA‐extractable pools of the above proteins, membranes from lung, heart, and brain contain EGTA‐resistant annexins V and VI which can be solubilized with detergents (Triton X‐100 or Triton X‐114). A strong base like Na2CO3, which is usually effective in extracting peripheral membrane proteins, only partially solubilizes the membrane‐bound, EGTA‐resistant annexins analyzed here. Also, only 50–60% of the Triton X‐114‐soluble annexins partition in the aqueous phase, the remaining fractions being recovered in the detergent‐rich phase. Altogether, these findings suggest that, by an as yet unknown mechanism, following Ca2+‐dependent association of annexin V isoforms and annexin VI with membranes, substantial fractions of these proteins remain bound to membranes in a Ca2+‐independent way and behave like integral membrane proteins. These results further support the possibility that the above annexins might play a role in membrane trafficking and/or in the regulation of the structural organization of membranes.