A LOW-MOLECULAR-MASS GTP-BINDING PROTEIN IN THE CYTOSOL OF GERMINATEDWHEAT EMBRYOS

A low-molecular-mass protein able to bind GTP in both native and SDS-d enaturating conditions was detected in the cytosol of embryos from whe at (Triticum aestivum L.) seeds germinated for 40 h. The protein fulfi lled most of the distinguishing criteria common to eukaryotic small GT P-binding proteins. It retained the ability to bind GTP after SDS/PAGE and nitrocellulose blotting. The protein eluted from Sephadex G-200 g el filtration with a V-e/V-o value corresponding to a molecular mass o f 18 kDa, whereas on SDS/PAGE the molecular mass was 20 kDa. The nativ e protein. which showed an intrinsic GTPase activity highly sensitive to NaF, bound the guanine nucleotide with high specificity and with a relatively high affinity (K-d approximate to 85 nM). The GTP-binding p rotein was not detectable in other subcellular fractions; in the micro somal fraction, two other peptides of low molecular mass (23.5 and 21. 5 kDa) with GTP-binding activity were detected. These results indicate that in the cytosolic fraction of germinating wheat embryos then is a 20-kDa protein which is biochemically similar to the known small GTP- binding proteins that currently have been detected almost exclusively in the membrane fraction of plant material.