Figure 2. Phospho-S680 ZEN-4 Localizes to the Central Spindle in Anaphase and to Remnants in Late Cytokinesis, and a Nonphosphorylatable S680A Mutant of ZEN-4 Is Partially Functional In Vivo. (A) Localization of phospho-S680 ZEN-4 in C. elegans embryos was analyzed by immunofluorescence. The color images show the position of the fluorescence in relation to the spindle, and the monochrome images quantitatively show the signal intensity. ZEN-4 staining can be observed on the central spindle during anaphase and at remnants ([a] and [b], arrow; arrowhead). Phospho-S680 ZEN-4 staining can also be observed at the central spindle prior to furrow ingression ([a'], arrow) and at division remnants ([b'], arrowhead). To determine specificity of the phospho-CeMKLP1 antibody staining, we preincubated the antibody with either the phospho-peptide that the antibody was raised against or the corresponding non-phospho-peptide (7.4 μg/ml). Central spindle staining in anaphase persists after antibody preincubation with the non-phospho-peptide ([c], arrow) but not with the phospho-peptide ([d], arrow). Nonspecific centrosome staining persists after phospho-peptide incubation ([d], white arrowheads).(B) Simple, extrachromosomal transgenic arrays marked by
rol-6(
su1006) were generated in MG 376, which carries the
zen-4 (
w35) null allele, and the rescue efficiency was determined for five independent lines either expressing ZEN-4 WT or four lines expressing ZEN-4 S680A. Wild-type
zen-4 rescues effectively the
zen-4 null background with an efficiency of 69%, whereas the nonphosphorylatable
zen-4 reduces the rescue efficiency to 18%. Error bars show the standard deviation. See Table S1 for details.(C) Complex extrachromosomal transgenic arrays marked by
rol-6(
su1006) were generated; these arrays rescue
zen-4 (
w35). ZEN-4::GFP localizes to the central spindle in anaphase (upper panel, 0 min) and localizes to the midbody and remnant in later stages of cytokinesis until sister-cell separation has been completed (upper panel, 6 min, 9 min, and 13 min). ZEN-4 S680A::GFP localizes to the central spindle in anaphase (lower panel, 0 min) and localizes to the midbody and early remnant (upper panel, 6 min and 9 min), but with furrow regression (upper panel, 9 min), ZEN-4 S680A::GFP moves with the regressing furrow (upper panel, 13 min) to the cortex (arrow).(D) Quantification of the frequency of failures in the first division of lines either expressing wild-type ZEN-4::GFP or ZEN-4 S680A::GFP.