Polymerization polarity of actin.

The actin polymer, like the thin filaments of muscle, is known to be polarized as demonstrated by the well known "HMM decoration" technique to give a "herringbone" pattern pointing in one direction. The question "Does polymer formation proceed unidirectionally, or bidirectionally?" was raised and tested experimentally. Short fragments of actin polymers were prepared, fully decorated with HMM and these decorated fragments were used as initiation centers for further actin polymerization without HMM. The resultant polymers showing both decorated and undecorated portions were examined and found to consist of a large majority of "spears" i.e., the added undecorated polymer extended in the direction opposite to that direction pointed by the "herringbone" pattern. However, a few cases of polymers indicating the opposite direction of polymerization were also found. Analysis leads to the conclusion that actin polymerization is unidirectional, although further experimentation is necessary to establish this completely.