Behavior of C-shaped microtubule endings in the cell

The association of incomplete microtubule assemblies with either another incomplete structure or complete microtubules was studied in two organisms, the phytoflagellate Polytoma papillatum and the phorid fly Megaselia scalaris, using transmission electron microscopy. In the alga, hook-shaped appendages on cytoplasmic and spindle microtubules were detected. These resulted from the lateral association of a curved ribbon of protofilaments with the surface of a complete microtubular wall. In the fly, an S-shaped protofilament sheet was found embedded in the kinetochore plate of a prometaphase I spermatocyte. Tracing of the S-shaped element towards the spindle pole revealed that it was formed by the lateral junction of two curved protofilament sheets. In all cases, the C-shaped protofilament sheets represented the endings of complete microtubules. Incomplete microtubules are generally considered as representing intermediates of microtubule assembly and disassembly. Since high molecular weight proteins are believed to be responsible for maintaining microtubule-microtubule spacing, it is hypothesized that the endings of growing and shrinking microtubules are sparsely studded with these proteins; their depletion allows lateral microtubule contacts. In addition, the microtubule-microtubule contacts may be rendered possible by the flexibility of the slender elongated microtubule-associated molecules. Relatively long C-shaped protofilament appendages (0.6-1.4 microns) were detected in this study. Therefore, it is plausible to assume that the protofilament sheets are stabilized by contact with one another or with an intact tubule wall.