a Related Genes Code for Functionally Related Proteins.

Multiple forms of tubulin, the major component of microtubules, have been identified by biochemical criteria. Two related forms of this protein, α- and β-tubulin have been identified (for review see Raff, 1979). As isolated from chick brain these forms do not differ in molecular weight, but can be distinguished on the basis of peptide maps and isoelectric points (Nelles and Bamburg, 1979). The structural similarity of these two tubulin proteins suggests that genes coding for them arose from a common precursor gene (Cleveland et al., 1980). DNA sequences containing genes coding for α- and β-tubulins have been identified using cDNA probes for them prepared from mRNA of chick brain, where tubulin represents % of the total cellular protein. Digestion of chick DNA with several restriction endonucleases reveals the presence of four unique fragments, which hybridize with both the 3′ and 5′ ends of each of the probes. This provides strong evidence that at least four separate genes code for each form of tubulin. Comparable analysis of human and rodent DNA reveals about ten genes coding for each form. Thus, what appeared to be two related proteins by biochemical criteria has been resolved into a family of eight to twenty related proteins by genetic criteria.

The nicotinic acetylcholine receptor, which mediates membrane events in synaptic transmission, represents another protein critical to nerve function for which two forms, junctional and extrajunctional, have been identified. Although in rat muscle these types of receptors can be distinguished on the basis of their affinities for d-tubocurarine, isoelectric points, and immunologic properties, they have the same subunit composition, and the subunits derived from them yield indistinguishable peptide maps (Nathanson and Hall, 1979). It is still not clear whether subunits of these two receptors originate from separate gene loci or represent different post-translational modifications of the same protein. Messenger RNA isolated from the electric organ of Torpedo can be used to direct the in vitro synthesis of polypeptides that cross-react with antibodies prepared against the acetylcholine receptor. Some of these newly synthesized polypeptides have apparent molecular weights different from the subunits of the receptor (Mendez et al., 1980). These data are consistent with the known post-translational modifications of these subunits (Raftery et al., 1980). Amino acid sequencing of the five subunits of the Torpedo receptor shows that two are identical and all are structurally related, suggesting that they arose from a common precursor gene. Analysis of the number and structure of genes coding for the mammalian acetylcholine receptor may provide insight into the relationship between junctional and extrajunctional receptors.

Monoamine oxidase, which degradatively deaminates biogenic amines throughout the body, also has two distinct functional types. The A type of MAO activity has a higher affinity for serotonin and norepinephrine and is inhibited by lower concentrations of clorgyline; while the B type has a higher affinity for phenylethylamine and benzylamine and is selectively sensitive to low concentrations of deprenyl (Murphy, 1978; Houslay et al., 1976). The flavin containing polypeptides of MAO-A and -B, isolated from rat and human sources, are structurally different on the basis of apparent molecular weight and peptide maps (Callingham and Parkinson, 1979; Cawthon et al., 1981; Brown et al., 1980; Cawthon and Breakefield, 1979). It is still not clear whether these structural differences result from variations in the primary amino acid sequence of different polypeptides or from post-translational modifications of the same polypeptide. For enzymes such as MAO, which are difficult to purify and for which there is no readily available source of mRNA, techniques of somatic cell genetics provide a means to resolve this issue (see Section II,B,1).

b Related Genes Code for Functionally Distinct Proteins.

A number of genes, which presumably have originated from gene duplications and evolutionary divergence in DNA sequence, can give rise to polypeptides that have substantial homology, but differ dramatically in function. Many “families” of related peptide hormones which serve both neural and endocrine functions have been described (Dockray, 1979; Stewart and Channabasavaiah, 1979). One family, the insulin-related polypeptides, includes insulin, relaxin, the insulin-like growth factors (Blundell and Humbel, 1980), and NGF (Bradshaw et al., 1974). Preliminary studies (L....