Dictyostelium discoideum SecG interprets cAMP‐mediated chemotactic signals to influence actin organization

Abstract

Tight control of actin cytoskeletal dynamics is essential for proper cell function and survival. ARNO, a mammalian guanine nucleotide exchange factor for Arf, has been implicated in actin cytoskeletal regulation but its exact role is still unknown. To explore the role of ARNO in this regulation as well as in actin-mediated processes, the Dictyostelium discoideum homolog, SecG, was examined. SecG peaks during aggregation and mound formation. The overexpression of SecG arrests development at the mound stage. SecG overexpressing (SecG OE) cells fail to stream during aggregation. Although carA is expressed, SecG OE cells do not chemotax toward cAMP, indicating SecG is involved in the cellular response to cAMP. This chemotactic defect is specific to cAMP-directed chemotaxis, as SecG OE cells chemotax to folate without impairment and exhibit normal cell motility. The chemotactic defects of the SecG mutants may be due to an impaired cAMP response as evidenced by altered cell polarity and F-actin polymerization after cAMP stimulation. Cells overexpressing SecG have increased filopodia compared to wild type cells, implying that excess SecG causes abnormal organization of F-actin. The general function of the cytoskeleton, however, is not disrupted as the SecG OE cells exhibit proper cell-substrate adhesion. Taken together, the results suggest proper SecG levels are needed for appropriate response to cAMP signaling in order to coordinate F-actin organization during development.

ATP levels influence cell movement during the mound phase in Dictyostelium discoideum as revealed by ATP visualization and simulation

“…To date, the involvement of cAMP in the centripetal migration of ATP‐rich cells remains unclear. However, it is known that cAMP signaling affects many ATP‐dependent events, such as cell shape alterations [ 53 , 54 ], actin polymerization [ 55 , 56 ], intracellular activation of several protein kinases [ 57 , 58 , 59 ], and changes in gene expression [ 33 , 60 ], suggesting that cAMP may regulate the behavior of ATP‐rich cells through these processes.…”

The C Isoform of Dictyostelium Tetraspanins Localizes to the Contractile Vacuole and Contributes to Resistance against Osmotic Stress

“…Cell-substrate adhesion was performed as described by Garcia et al . [ 31 ] with slight modifications. A suspension of 1 × 10 6 cells in 1 ml of SPB was placed in a 125 ml glass culture flask and put on a gyratory shaker for 10 min at 120 rpm.…”