Myosin Va and kinesin II motor proteins are concentrated in ribosomal domains (periaxoplasmic ribosomal plaques) of myelinated axons.

Periaxoplasmic ribosomal plaques (PARPs) are discrete ribosome-containing domains distributed intermittently along the periphery of axoplasm in myelinated fibers. Thus, they are structural formations in which translational machinery is spatially organized to serve as centers of protein synthesis for local metabolic requirements and perhaps repair as well. Because of evidence that RNA is transported to putative PARP domains, involving both microtubule- and actin-based mechanisms, it was of interest to investigate whether cytoskeletal motor proteins exhibit a nonrandom localization within PARP domains. Axoplasm, from large Mauthner fibers and rat or rabbit spinal ventral nerve root fibers, removed from the myelin sheath in the form of an "axoplasmic whole-mount" was used for this analysis. PARP domains were identified either by specific immunofluorescence of rRNA, ribosomal P antigen, or by nonspecific RNA fluorescence using RNA binding dyes YOYO-1 or POPO-1. A polyclonal antibody (pAb) against the motor domain of myosin Va showed prominent nonrandom immunofluorescence labeling in PARP domains. Similarly, monoclonal antibodies (mAb) against kinesin KIF3A and a pan-specific antikinesin (mAb IBII) also showed a preponderant immunofluorescence in PARP domains. On the other hand, H2, a mAb antikinesin KIF5A, exhibited only random immunofluorescence labeling in axoplasm, as was also the case with pAb antidynein heavy chain immunofluorescence. Several possible explanations for these findings are considered, primary among which is targeted trafficking of translational machinery that results in local accumulation of motor proteins. Additional possibilities are trafficking functions intrinsic to the domain, and/or functions that govern dynamic organizational properties of PARPs.