Actin-dependent myoid elongation in teleost rod inner/outer segments occurs in the absence of net actin polymerization.
In the retinas of teleost fish, rod photoreceptors elongate in response to light. Light-activated elongation is mediated by the myoid of the rod inner segment and is actin-dependent. Inner segment F-actin filaments form bundles running parallel to the cell's long axis. We examined the mechanism of rod elongation using mechanically-detached rod fragments, consisting of the motile inner segment and sensory outer segment (RIS-ROS). When RIS-ROS are isolated from dark-adapted green sunfish and cultured in the light, they elongate 15 microns at 0.3-0.6 microns/min. Elongation was inhibited 65% by 0.1 microM Cytochalasin D, suggesting a requirement for actin assembly. To determine the extent of assembly during elongation, we used three approaches to measure the F-actin content in RIS-ROS: detection of pelletable actin by SDS-PAGE after detergent-extraction of RIS-ROS; quantification of fluorescein-phalloidin binding by fluorimetry, fluorescence-activated cell sorting and image analysis; estimation of total F-actin filament length by electron microscopy. All three assays indicated that no net assembly of RIS-ROS F-actin accompanied myoid elongation. An increase in F-actin content within the elongated myoid was counterbalanced by a decrease in F-actin content within the 13 microvillus-like calycal processes located at the end of the inner segment opposite to the growing myoid. O'Connor and Burnside (Journal of Cell Biology 89:517-524, 1981) showed that minus-ends of rod F-actin filaments are oriented towards the elongating myoid while plus-ends are oriented towards the shortening calycal processes. Our observations suggest that RIS-ROS elongation entails actin polymerization at the minus-ends of filaments coupled with depolymerization at the filament plus-ends.
Pagh-Roehl
K
K
Department of Molecular & Cell Biology, University of California, Berkeley 94720.
Brandenburger
J
J
Wang
E
E
Burnside
B
B
D008841:Actin Cytoskeleton / Q000502:physiology / Q000648:ultrastructure; D000199:Actins / Q000032:analysis; D000818:Animals; D002465:Cell Movement / Q000187:drug effects / Q000502:physiology*; D002469:Cell Separation; D015638:Cytochalasin D / Q000494:pharmacology; D003599:Cytoskeleton / Q000502:physiology* / Q000648:ultrastructure; D005399:Fishes / Q000502:physiology*; D005455:Fluorescent Antibody Technique; D008854:Microscopy, Electron; D009024:Morphogenesis / Q000187:drug effects / Q000502:physiology*; D010590:Phalloidine / Q000378:metabolism; D010775:Photic Stimulation; D010786:Photoreceptor Cells / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000502:physiology / Q000648:ultrastructure
D000199:Actins; D010590:Phalloidine; D015638:Cytochalasin D