These studies show that the antibodies cross react with all classes of GPs and thus the immunodetection method is far more versatile as compared to the other two methods. Most of the low Mr GPs were detectable when GTP-binding method was used, while only a subset of high Mr GPs were detectable on the basis of CT-enhanced ADP ribosylation. Most of the GPs seem to be conserved and only a few are species-specific. Whereas corn shows a single high Mr GP (56-60 kDa), sorghum contained three different ones of which two were also ADP-ribosylated. The ADP-ribosylation of the -subunit of heterotrimeric GPs is well known and such heterotrimeric GPs are localized in the plasmamembrane. In the present studies these GPs were found to be present not only in the microsomal pellet but also with the cytosolic and organellar fractions. At least four different ones were observed in corn and of these two were ADP ribosylated in the presence of CT. The sizes of corn GPs found in present studies are comparable to that reported earlier (see Ma, Plant Mol. Biol. 26:1611, 1994). The sorghum on the other hand has only three types of -subunits.
We are trying to determine if the low Mr GPs belong to the Ras, Rho or Ypt/Rab class, which are known to play a role in secretion, intracellular transport or the restructuring of cytoskeleton during development and differentiation. In addition to the -subunit of heterotrimeric GPs, the present studies suggest that some of the GPs of still higher Mr (56-60 and 68 kDa) can also be ADP-ribosylated in vitro. High Mr GPs have earlier been reported in mammalian systems; for instance, a 67 KDa GP has been reported to be coupled to insulin receptor and plays a role in the signal transduction process. A similar function for the higher Mr GP in plants cannot be ruled out.
Table 1. Different types of GTP-binding proteins from corn and sorghum.
|Size (KDa)||Detection methods||Sub-cellular localization||Plant species|
|56-60||Ab, AR||Organelles||C, S|
|44-45||Ab, AR||Microsomes and cytosol||C, S|
|41-43||Ab, AR||Organelles, microsomes and cytosol||C, S|
|35-38.5||Ab||Microsomes and cytosol||C, S|
|32||Ab||Microsomes and cytosol||C|
|29,26,18||Ab, 32P-GTP||Chloroplasts||C, S|
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