Experiments were designed to determine the types of high molecular weight (HMW) MAPS present in brain tissue of the smooth dogfish shark (Mustelus canis) and the optic ganglia of the squid (Loligo pealei). We previously reported' that microtubule proteins (MTPs) from dogfish brain, purified by temperature cycling, showed a lack of HMW MAPs. In subsequent studies, we have found that the temperature at which the microtubules are assembled significantly influences the retention of MAPS during purification. We found that when microtubules are assembled at 30°C rather than at 18-22"C (the physiological temperatures for these organisms), HMW MAPS were consistently observed after two or three cycles of purification. Therefore, we have used 30°C as the assembly temperature in these studies. Samples of dogfish brain MTPs, after one cycle of assembly-disassembly, were run on sodium dodecyl sulfate (SDS) polyacrylamide gels. Three prominent HMW bands were seen (FIGURE 1, lanes 3 and 4). The slowest migrating HMW band ran slightly slower than cow brain MAP-1 (FIGURE 1, lanes 5 and 6) and is presumed to be omologous to MAP-1. The fastest migrating HMW band ran faster than cow brain MAP-2 and is presumed to be its homologue. The middle band was a minor HMW protein that migrated between MAPS 1 and 2. MAP-1 of dogfish brain (FIGURE 1, lane 10) was found to be heat labile after boiling at 100°C for 5 min (FIGURE 1, lanes 11 and 12). Tau-like proteins that comigrated with the tau proteins of cow brain (FIGURE 1, lanes 8 and 9) were seen on gels of dogfish brain MTP amples (FIGURE 1, lanes 11 and 12) after heat treatment. Experiments were also designed to determine if differences in the relative amounts of the HMW MAPS were apparent in different regions of the dogfish central nervous system (CNS). Three regions of the CNS (designated brain part I, brain part 11, and the spinal cord) were investigated. Brain part I (the telen-, mesen-, and diencephalon) was highest in gray matter. Brain part II (rhombencephalon and corpus cerebelli) had similar amounts of gray and white matter, and the spinal cord (a 10-12 cm segment from the anteriormost portion of the cord) was highest in white matter. Densitometic scans of the gel lanes (FIGURE 2) showed that after one cycle of assembly, brain part I had a higher amount of MAP-2 than MAP-1 (FIGURE 2, lane 1). The high amount of MAP-2 in brain part I may reflect the high percentage of gray matter in this part of the brain. The relative amounts of MAPS 1 and 2 in brain part II "This work was supported by NIH Grant GM-28107