A membrane protein insoluble in water was isolated by gel chromatography in the presence of 0.1% sodium dodecyl sulfate (SDS) from chromatophores of a photosynthetic bacterium, Rhodospirillum rubrum. This is one of the major membrane proteins of the chromatophore. The protein was found to bind about four grams of SDS per gram, a value which is more than twice the amount generally observed with protein polypeptides derived from water-soluble globular proteins. The electrophoretic behavior of the complex between the membrane protein and SDS is abnormal due to this high capacity for binding SDS. Estimation of the molecular weight of this protein by SDS-polyacrylamide gel electrophoresis was thus impossible. Such an anomaly in SDS binding is unlikely to be restricted to the particular membrane protein described in this paper. The possibility of such a deviation from standard behavior in the interaction with SDS should be taken into consideration in studies of other membrane proteins, since SDS is often used both in analytical and preparative procedures.

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1.A factor, which makes a ternary complex with GTP and eukaryotic initiator tRNA (MettRNAi), has been purified 100-fold from developed cysts of Artemia salina. Some of the properties of the purified factor have been studied.2. M 2 + appears to inhibit ternary complex formation. 3. Little or no ternary complex is formed when 5 pM GTP is replaced by an identical concentration of UTP, CTP or ATP. The analog, guanosine 5'-(fi,y-imino)triphosphate [GMP-P(NH)P] seems to be a much better substitute for GTP than guanosine 5'-(B~-methylene)triphosphate [GMP-P(CH2)PI. Since GMP-P(NH)P is as effective as GTP in ternary complex formation, it would appear that GTP plays the role of an allosteric effector in this step of eukaryotic polypeptide chain initiation.4. GDP inhibits both the rate and extent of ternary complex formation. The inhibition is largely reversed by adding a 5-fold molar excess of GTP over GDP. dGDP is slightly less inhibitory than GDP. UDP and CDP are much less inhibitory than GDP and very little inhibition is obtained with ADP.5. The preformed ternary complex is rapidly and completely destroyed in the presence of N-ethylmaleimide. The results suggest that free -SH groups of the factor may be essential for maintaining the integrity of the ternary complex.Despite much work on the mechanism of polypeptide chain initiation in eukaryotic cells, relatively little information is presently available regarding the precise role of GTP in this process. For example, our understanding of the step(s) in which GTP plays an allosteric role, of the step(s) in which GTP is hydrolyzed and of the role of GTP hydrolysis in eukaryotic polypeptide chain initiation, is either incomplete or lacking. In order to clarify the above problems, we have recently undertaken a project directed toward the purification and detailed examination of the properties of eukaryotic protein factor(s) whose functioning in the process of polypeptide chain initiation is either dependent on and/or modified by guanine nucleotides. In the present paper, a procedure is described for 100-fold purification of such a factor from developed cysts of the brine shrimp, Artemiu Ahhrrviations. GMP-P(CH2)P and GMP-P(NH)P, guanosine S'-(p,.;-methylene)triphosphate and guanosine S'-(p,y-imino)triphosphate, respectively; IF-2, prokaryotic initiation factor 2; MalNEt, N-ethylmaleimide, -~ ~ salina. The purified factor forms a ternary complex with GTP and initiator Met-tRNA (Met-tRNAi) of eukaryotic origin. The role of GTP in this reaction appears to be that of an allosteric effector since the analog GMP-P(NH)P is as effective as GTP in ternary complex formation. GMP-P(CH2)P seems to be much less active than GMP-P(NH)P. Although GDP inhibits ternary complex formation, the inhibition is, reversed by increasing concentrations of GTP. We have also observed that the preformed factor . GTP . MettRNAi ternary complex is rapidly destroyed in the presence of N-ethylmaleimide. These and other properties of the purified faFtor are described in this paper.Previous work from other l...