Differential mechanisms of retinoid transfer from cellular retinol bindingproteins types I and II to phospholipid membranes

Cellular retinol-binding proteins types I and LI (CRBP-I and CRBP-II) are k nown to differentially facilitate retinoid metabolism by several membrane-a ssociated enzymes. The mechanism of ligand transfer to phospholipid small u nilamellar vesicles was compared in order to determine whether differences in ligand trafficking properties could underlie these functional difference s. Unidirectional transfer of retinol from the CRBPs to membranes was monit ored by following the increase in intrinsic protein fluorescence that occur s upon ligand dissociation, The results showed that ligand transfer of reti nol from CRBP-I was >5-fold faster than transfer from CRBP-II, For both pro teins, transfer of the other naturally occurring retinoid, retinaldehyde, w as 4-5-fold faster than transfer of retinol, Rates of ligand transfer from CRBP-I to small unilamellar vesicles increased with increasing concentratio n of acceptor membrane and with the incorporation of the anionic lipids car diolipin or phosphatidylserine into membranes. In contrast, transfer from C RBP-II was unaffected by either membrane concentration or composition. Prei ncubation of anionic vesicles with CRBP-I was able to prevent cytochrome c, a peripheral membrane protein, from binding, whereas CRBP-II was ineffecti ve. In addition, monolayer exclusion experiments demonstrated differences i n the rate and magnitude of the CRBP interactions with phospholipid membran es. These results suggest that the mechanisms of ligand transfer from CRBP- I and CRBP-II to membranes are markedly different as follows: transfer from CRBP-I may involve and require effective collisional interactions with mem branes, whereas a diffusional process primarily mediates transfer from CRBP -II. These differences may help account for their distinct functional roles in the modulation of intracellular retinoid metabolism.