Differential compartmentalization of mRNAs in squid giant axon

Previously, we reported that the squid giant axon contains a heterogeneous population of mRNAs that includes beta-actin, beta-tubulin, kinesin, neurofilament proteins, and enolase. To define the absolute levels and relative distribution of these mRNAs, we have used competitive reverse transcription-PCR to quantify the levels of five mRNAs present in the giant axon and giant fiber lobe (GFL), the location of the parental cell soma. In the GFL, the number of transcripts for these mRNAs varied over a fourfold range, with beta-tubulin being the most abundant mRNA species (1.25 x 10(9) molecules per GFL). Based on transcript number, the rank order of mRNA levels in the GFL was beta-tubulin > beta-actin > kinesin > enolase > microtubule-associated protein (MAP) H1. In contrast, kinesin mRNA was most abundant in the axon (4.1 x 10(7) molecules per axon) with individual mRNA levels varying 15-fold. The rank order of mRNA levels in the axon was kinesin > beta-tubulin > MAP H1 > beta-actin > enolase. The relative abundance of the mRNA species in the axon did not correlate with the size of the transcript, nor was it directly related to their corresponding levels in the GFL. Taken together, these findings confirm that significant amounts of mRNA are present in the giant axon and suggest that specific mRNAs are differentially transported into the axonal domain.