Cohesive axonal transport of the slow component b complex of polypeptides

Abstract

The axonal transport of the diverse group of polypeptides characteristic of slow component b (SCb), or Group IV, may be accomplished either by a bulk flow mechanism acting on each of the individual polypeptides or by their movement through the axoplasm as part of a physical complex or structural assembly. In order to determine if the SCb polypeptides travel as a cohesive unit (which would be consistent with the latter alternative above), we examined in detail the quantitative distribution of the individual SCb polypeptides in the guinea pig optic system at three different times after pulse radiolabeling the neuron cell bodies. Eighteen SCb polypeptides were selected as appropriate for analysis. Individual optic systems at the three select times after labeling were harvested, cut into small segments, and subjected to SDS-PAGE. Individual radioactive polypeptide bands were identified and located by fluorography, excised, and then quantified by scintillation counting. The results indicate that the SCb polypeptides travel cohesively: they all have nearly superimposable distribution profiles at the advancing “front” of the SCb wave of radioactivity. However, the distribution profiles are not identical: there is considerable variation among the polypeptides in the portion of the distribution profiles trailing behind the advancing peak and in the breadth of the peak’s crest. This variation is consistent with the hypothesis that the individual polypeptides have differing affinities for an SCb group, or structure, as it moves down the axon. The majority of material transported anterogradely in axons is transported within three distinct axonal transport components. These three components of axonal transport were characterized initially by the rate at which they traversed the axons (as in Grafstein and Forman, 1980); however, recently, the idea has emerged that they also may be structurally and functionally distinct entities (Droz et al., 1975; Lorenz and Willard, 1978; Lasek, 1980; Tytell et al., 1981). The fast component of axonal transport (moving at 250 to 400 mm/day) and the slower component (SCa, moving at 0.2 to 1 mm/day) have both been relatively well characterized by both rate determinations and macromolecular constituents (Ochs, 1972; ’ We gratefully acknowledge the excellent technical assistance of Shirley Ricketts and Diane Filsinger, the assistance of Michael Lark with the computations, and the secretarial skills of Mrs. Ann Martin during the completion of this work. This study was supported by the National Institutes of Health Predoctoral Fellowship HD-07104 to J. A. G. and National Institutes of Health Grants NS 14900-02 and NS 13658-03 to R. J. L. This work has been presented in preliminary form (Garner and Lasek, 1978). ’ Present address: Department of Chemistry, Brain Research Group, Indiana University, Bloomington, IN 47405. Hoffman and Lasek, 1975; Willard et al., 1974; Willard and Hulebak, 1977; Tytell et al., 1981). The fast component contains primarily glycoproteins (Forman et al., 1972), membranes and membrane vesicles (Droz et al., 1975; Tsukita and Ishikawa, 1980), or the materials related to transmitter release (Schwartz et al., 1976; Brimijoin, 1975). SCa, on the other hand, is composed primarily of cytoskeletal proteins, including tubulin and the neurofilament-associated proteins (Hoffman and Lasek, 1975; Lasek and Hoffman, 1975). Its structural counterpart has been suggested to be the axonal cytoskeleton (Lasek, 1980). The third major axonal transport component (at 2 to 4 mm/day) is the component designated SCb (Black and Lasek, 1979, 1980; Brady and Lasek, 1981; Garner and Lasek, 1981) or the Group IV polypeptides (Willard et al., 1974). SCb is extremely complex in constitution, the identity of its constituents ranging from microfilamentassociated proteins, such as actin (Black and Lasek, 1979; Willard et al., 1979), the myosin-like M-2 protein (Willard, 1977), and clathrin (Garner and Lasek, 1981), to many of the metabolic enzymes, such as creatine phosphokinase, nerve-specific enolase, and calmodulin (Brady and Lasek, 1981; Brady et al., 1981).