We have developed microdensitometer-computer correlation techniques to analyze the arrangement of microtubule arms and bridges (i.e., microtubule-associated proteins [MAPs]). A microdensitometer was used to scan immediately adjacent to the wall of longitudinally sectioned microtubules in positive transparency electron micrographs. Signal enhancement procedures were applied to the digitized densitometer output to produce a binary sequence representing the apparent axial spacing of MAP projections. These enhanced records were analyzed in two ways. (a) Autocorrelograms were formed for each record and correlogram peaks from a group of scans were pooled to construct a peak frequency histogram. (b) Cross-correlation was used to optimize the match between each enhanced record and templates predicted by different models of MAP organization. Seven symmetrical superlattices were considered as well as single axial repeats. The analyses were repeated with randomly generated records to establish confidence levels. Using the above methods, we analyzed the intrarow bridges of the Saccinobaculus axostyle and the MAP2 projections associated with brain microtubules synthesized in vitro. We confirmed a strict 16-nm axial repeat for axostyle bridges. For 26 MAP2 records, the only significant match was to a 12-dimer superlattice model (P less than 0.002). However, we also found some axial distances between MAP2 projections which were compatible with the additional spacings predicted by a 6-dimer superlattice. Therefore, we propose that MAP2 projections are arranged in a "saturated 12-dimer, unsaturated 6-dimer" superlattice, which may be characteristic of a wide variety of MAPs.

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