A Compendium of Characteristics for the Rapidly-Metabolized 32 Kd Protein of the Chloroplast Membrane
Our studies have concentrated on a 32 kd protein partially surface exposed at the photosynthetic membranes of the aquatic angiosperm, Spirodela oligorrhiza. This protein is encoded and translated within the chloroplast as a 33.5 kd precursor polypeptide. Processing to the 32 kd form occurs in vivo on the membrane and commences only after completion of the 33.5 kd polypeptide chain . Polypeptide fragments, obtained following sequential proteolyses by several enzymes, have been aligned to give a proteolytic map of the 33.5 kd precursor protein. The short maturation fragment was found at the end of the hydrophylic portion of the molecule, which extends beyond the thylakoid membrane. In comparisons involving rapidly-labelled 32 kd polypeptides from chloroplast membranes of various angiosperms and the alga Chlamydomonas, a broad distribution and high degree of similarity was found at levels of precursor maturation, membrane orientation and primary structure . On the other hand, a number of surface-exposed thylakoid polypeptides exist, at various levels of abundancy, with mobilities in SDS-polyacrylamide gels very similar to that of the rapidly-metabolized 32 kd polypeptide [1,13,16,21,23,30]. The protein we are studying is the product of the “32 kd gene” mapping in the large, unique-copy region of chloroplast DNA adjacent to one of the inverted repeats . A list of distinguishing characteristics for the rapidly-metabolized 32 kd protein is given in Table I. It is hoped that this will be of some help to those faced with problems of polypeptide identification.
KeywordsThylakoid Membrane Inverted Repeat Chloroplast Membrane Chloroplast Development Photosynthetic Membrane
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