Model of the molecular mechanisms that underlie cp-actin-based chloroplast movements. Model of the molecular mechanisms that underlie cp-actin-based chloroplast movements. The phot2 protein, which functions as a photoreceptor for the avoidance response, localizes to the outer envelope of the chloroplast. Phot1 and phot2, which mediate the accumulation response, localize to the plasma membrane. Signals for the accumulation and avoidance response are released from the photoreceptors and are received by a signal receptor that is probably associated with CHUP1, which anchors the chloroplast to the plasma membrane through an unknown membrane protein (labeled X). A potential CHUP1-dependent polymerization of cp-actin filaments is initiated by the incorporation of profilin-bound actin. KAC acts to promote cp-actin polymerization together with CHUP1 or in the maintenance of cp-actin filaments, at least in the accumulation response. Polymerized cp-actin filaments are bundled by THRUMIN1, which localizes to the plasma membrane, so that cp-actin filaments are fixed to the plasma membrane. Thus, the chloroplast slides forward by newly polymerized cp-actin as long as cp-actin is polymerized. Other factors (e.g. PMI1 and WEB1) that are believed to regulate cp-actin dynamics are not included in this model because their exact molecular functions and the relationship with other factors are unclear. Masamitsu Wada, and Sam-Geun Kong J Cell Sci 2018;131:jcs210310 © 2018. Published by The Company of Biologists Ltd