Topographical Studies on Subunit Polypeptides of the PS I Reaction Center Complex in the Thylakoid Membranes of the Thermophilic Cyanobacterium Synechococcus sp.

Publisher Website
Google Scholar
Abstract
The permeability to protein molecules of the outer limiting membranes and the thylakoid membranes in hypotonically shocked protoplasts of the thermophilic cyanobacterium Synechococcus sp. was studied by examining the effects of NaBr-washing and pronase E-digestion on phycobiliproteins and a 35 kDa protein which are associated with the outer and inner surface of the thylakoid membranes, respectively, and by measuring photooxidation of added cytochrome c. All the results obtained indicate that the shocked protoplasts are in essence a homogenous right side-out thylakoid membrane preparation; the outer limiting membranes are leaky to protein molecules, whereas the thylakoid membranes are still impermeable to proteins. The thylakoid membranes became permeable to proteins when the protoplasts were mechanically disrupted. Following on from these findings, the membrane topology of subunit polypeptides of the photosystem I reaction center complex was studied. Proteolytic digestion of shocked protoplasts with trypsin and pronase E indicated that four of the five subunit polypeptides of the PS I reaction center complex are exposed at the stromal surface of thylakoid membranes; two subunits of 14 and 13 kDa were selectively digested by trypsin, whereas two chlorophyll-binding subunits of 62 and 60 kDa were preferentially attacked by pronase E. However, a 10 kDa subunit appears to be strongly resistant to the proteases. Experiments with mechanically disrupted protoplasts failed to provide evidence for a uniform transmembrane organization of the PS I subunits.