Binding of ras p21 to bands 4.2 and 6 of human erythrocyte membranes

Heterogeneity of guanine nucleotide binding proteins in human red blood cell membranes

Membranes from human erythrocytes bind radioactive GTP and GTP analogs according to apparently homogeneous patterns. In spite of this uniform type of association, multiple guanine nucleotide binding proteins have been identified both by SDS-PAGE analysis of native and of variously ADP-ribosylated membrane preparations and by FPLC chromatography of solubilised erythrocyte membranes preliminarily incubated with [alpha-32P] GTP in the presence of 5 mM MgCl2. From eight to nine peak fractions of pronase-digestible GTP-binding activity were separated on a MA7Q anion exchange column, this pattern being highly reproducible with different membrane preparations. Prior incubation of membranes with [alpha-32P] GTP in the presence of excess unlabeled GDP resulted in displacement of bound labeled nucleotide from all FPLC fractions. The patterns of GTP binding were also markedly modified by preliminary treatment of membranes with N-ethylmaleimide. Detectable GTPase activity was present in each of the FPLC peak fractions. This wide heterogeneity of guanine nucleotide binding proteins raises so far unanswered questions as to their physiological significance in the mature erythrocyte.

Multiple small molecular weight guanine nucleotide-binding proteins in human erythrocyte membranes

Native membranes from human erythrocytes contain the following G proteins which are ADP-ribosylated by a number of bacterial toxins: Gi alpha and Go alpha (pertussis toxin), Gs alpha (cholera toxin), and three proteins of 27, 26 and 22 kDa (exoenzyme C3 from Clostridium botulinum). Three additional C3 substrates (18.5, 16.5 and 14.5 kDa) appeared in conditions of unrestrained proteolysis during hemolysis. SDS-PAGE separation of erythrocyte membrane proteins followed by electroblotting and incubation of nitrocellulose sheets with radiolabeled GTP revealed consistently four GTP-binding proteins with Mr values of 27, 26, 22 and 21 kDa. Although a 22 kDa protein was immunochemically identified as ras p21, the C3 substrate of 22 kDa is a different protein probably identifiable with a rho gene product. Accordingly, at least five distinct small molecular weight guanine nucleotide-binding proteins, whose functions are so far undetermined, are present in native human erythrocyte membranes.

Localization and subcellular distribution of cellular ras gene products in rat brain

Localization and subcellular distribution of the cellular ras gene products (c-ras p21s) in rat brain were studied by immunofluorescence and immunoblotting using a monoclonal antibody recognizing all of Ki-, Ha- and N-ras p21s. In immunohistochemical analysis, strong immunoreactivity for ras p21s was observed in the neuropile of cerebral and cerebellar cortex. On the other hand, the immunoreactivity of the neuronal perikarya and that of white matter were weak and that of non-neuronal cells was undetectable. In subcellular fractionation analysis of cerebrum, c-ras p21s were found mostly in the particulate fractions and almost half of the particulate-bound c-ras p21s were recovered in the P2 fraction containing myelin, synaptosomes and mitochondria, approximately one-third were in the P3 fraction containing microsomes, and the rest were in the P1 fraction containing nuclei and cell debris. In further fractionation of the P2 fraction, most of c-ras p21s were associated with synaptosomal fraction. In the synaptosomal fraction, c-ras p21s were highly concentrated in the fractions rich in synaptic plasma membranes and were poorly present in the other fractions rich in synaptic vesicles, intrasynaptosomal mitochondria or postsynaptic densities. The content of c-ras p21s of the original homogenate was calculated to be 0.05% of the total protein and c-ras p21s were distributed in the fractions rich in synaptic plasma membranes with approximately 4-fold enrichment over the original homogenate. These results indicate that c-ras p21s are mainly localized in the synaptic plasma membranes and microsomes and suggest that they may participate in some specific neuronal functions at these sites.

Small molecular weight GTP-binding proteins in human erythrocyte ghosts

GTP-binding proteins (G proteins) were extracted from human erythrocyte ghosts by sodium cholate and purified by gel filtration on an Ultrogel AcA-44 column followed by hydroxyapatite column chromatography. At least two peaks of G proteins were separated by hydroxyapatite column chromatography. The second peak contained G proteins recognized by the antibodies against the respective alpha subunits of Gs and Gi, and the ras protein, while the G protein of the first peak was not recognized by any of these antibodies. The G protein of the first peak was purified further by Mono Q HR5/5 column chromatography. The purified G protein showed a molecular weight of about 22 kDa on SDS-polyacrylamide gel electrophoresis. This G protein (22K G) specifically bound guanosine 5'-(3-O-thio) triphosphate (GTP gamma S), GTP and GDP with a Kd value for GTP gamma S of about 50 nM. GTP gamma S-binding to 22K G was inhibited by pretreatment with N-ethylmaleimide. The G proteins recognized by the antibodies against the alpha subunit of Go and the ADP-ribosylation factor for Gs, designated as ARF, were not detected in human erythrocyte ghosts. These results indicate that there are at least two species of small molecular weight G proteins in human erythrocyte ghosts: one is the ras protein and the other is a novel G protein of 22K G.