TABLE 2.

Fascin binding is selective for parallel actin filaments

Fascin only binds to filaments oriented in a parallel orientation (0°). Crossed filaments (16–165°) form no cross-links. Filaments oriented in an anti-parallel orientation do not form cross-links. When two beads are attached to the same filament, we observed 8 of 18 tests forming cross-links. In this orientation, approximately 50% of the trials should have filaments oriented in a parallel manner and 50% in an anti-parallel orientation. When two polarity-labeled filaments were allowed to interact in a parallel orientation, they were observed to form cross-links and bundles. From these results, we determined that fascin will only form cross-links when filaments are oriented in a parallel orientation. For crosses of 16–165°, a scan of one angle was performed, and because no cross-links were observed, a new angle was selected, and another scan was performed with that same filament pair. The notation 4/4 in the crosses tested field mean four crosses were formed, and each of those crosses was tested in both orientations represented in that bin on the table. The same meaning is intended for the 5/5 notation. NA, not applicable.

Angle (degrees)	Orientation	Orientation Fig. reference	Crosses tested	Crosses that bound	Probability of sampling only one conformation
0	Polarity labeled, four bead, two filaments	1F	2	2	1
180	Polarity labeled, four beads, two filaments	1F	2	0	1
0/180	One bead, two filaments	1I	18	8	7.6 × 10−6
180	One bead, wrapped filament	1G	9	0	1
16–45/36–165	Four beads, two crossed filaments	1D	4/4	0	NA, four crosses rotated through range
46–89/91–135	Four beads, two crossed filaments	1D	5/5	0	NA, five crosses rotated through range
90	Four beads, two crossed filaments	1D	5	0	1