INTRACELLULAR-LOCALIZATION OF TROPOMYOSIN MESSENGER-RNA AND PROTEIN IS ASSOCIATED WITH DEVELOPMENT OF NEURONAL POLARITY

Neuronal differentiation involves extensive rearrangement of the cytos keleton, including the actin-based microfilament system, and establish ment of molecular compartments within the neuron. The intracellular di stribution of tropomyosin (Tm) mRNA in vivo and in vitro has been exam ined and correlated with protein targetting. The mRNAs encoding two Tm isoforms were found to be differentially localized in developing neur ons. Tm-5 mRNA is localized to the axonal pole of differentiating embr yonic rat neurons, in contrast to TmBr-2 mRNA distribution throughout the cell body. Tm-5 mRNA is transported into the axon of differentiati ng primary cultured neurons. This mRNA localization is developmentally regulated and correlates with the targeting of Tm-5 protein to growin g axons. Tm-5 colocalizes with a subset of neuronal microfilaments ass ociated with the initiation and maintenance of outgrowth. The segregat ion of Tm-5 is the earliest known marker of neuronal polarity and may play a role in the establishment of polarity.