AtMAC stabilizes the phragmoplast by crosslinking microtubules and actin filaments during cytokinesis

The phragmoplast, a structure crucial for the completion of cytokinesis in plant cells, is composed of antiparallel microtubules (MTs) and actin filaments (AFs). However, how the parallel structure of phragmoplast MTs and AFs is maintained, especially during centrifugal phragmoplast expansion, remains elusive. Here, we analyzed a new Arabidopsis thaliana MT and AF crosslinking protein (AtMAC). When AtMAC was deleted, the phragmoplast showed disintegrity during centrifugal expansion, and the resulting phragmoplast fragmentation led to incomplete cell plates. Overexpression of AtMAC increased the resistance of phragmoplasts to depolymerization and caused the formation of additional phragmoplasts during cytokinesis. Biochemical experiments showed that AtMAC crosslinked MTs and AFs in vitro, and the truncated AtMAC protein, N-CC1, was the key domain controlling the ability of AtMAC. Further analysis showed that N-CC1(51-154) is the key domain for binding MTs, and N-CC1(51-125) for binding AFs. In conclusion, AtMAC is the novel MT and AF crosslinking protein found to be involved in regulation of phragmoplast organization during centrifugal phragmoplast expansion, which is required for complete cytokinesis.