Differential expression of ascorbate peroxidase and a putative molecular chaperone in the boundary membrane of differentiating cucumber seedling peroxisomes

In previous research, two integral membrane polypeptides, M-r 30,000 and 73,000, were shown to be localized exclusively to the boundary membrane of glyoxysomes in cotyledons of 2 d, dark-grown cucumber seedlings (Corpas et al., 1994). In subsequent studies, the M-r 30,000 polypeptide was discovered to be a glyoxysomal membrane ascorbate peroxidase (designated gmAPX), and the M-r 73,000 polypeptide was identified as a membrane-bound molecular chaperone (designated as gmHsp73) due to its immunorelationship with the 70 kDa family of heat shock proteins (Hsp70). To become knowledgeable of the role(s) of these polypeptides in cotyledon peroxisomes during later stages of seedling growth, we examined on immunoblots semi-quantitative changes in their amount during four stages of cucumber seedling growth. The gmHsp73 was not detected in peroxisomal or microsomal membrane fractions at any of the post-glyoxysomal stages indicating that it had a specific role in glyoxysomal Function. In contrast, gmAPX persisted in the peroxisome membranes at all stages, i.e. in transition peroxisomes (glyoxysomes to leaf-type peroxisomes, 2d dark, Id light), leaf-type peroxisomes (at least 3d light), and in senescent peroxisomes (gerontosomes, 21d light). Hence, this APX appears to be a constitutive enzyme in the different types of seedling peroxisomes likely protecting the boundary membrane from toxic H2O2 produced by the metabolic pathways in the differentiating peroxisomes. The designation of this APX is appropriately changed from gmAPX to pmAPX to reflect its common occurrence in the membrane of all types of peroxisomes.