Chemoattractant is used to demonstrate signal-induced gradient of Gp subunit. False color images of amebas expressing (a) a GFP-tagged Gp subunit and (b) a GFP-tagged cAMP receptor, which served as a control. When an external source of chemoattractant was placed near the cells (at the top of the photographs), the cells turned toward the source. (a) The Gp subunit became concentrated at the leading edge of the cell, closest to the chemoattractant, and depleted from the tail. (b) In contrast, the cAMP receptor retained a uniform distribution in the cell membrane. [W. F. Loomis and R. H. Insall, 1999, Nature 401:440-441; courtesy of Peter Devreotes Laboratory.]
leading to the activation of Arp 2/3 through the mediator protein WASp or through other pathways that increase cytosolic Ca2+ (see Figure 13-29).
Findings from studies with fluorescent dyes that act as internal Ca2+ sensors indicate that a cytosolic gradient of Ca2 + also is established in migrating cells, with the lowest concentration at the front of the cell and the highest concentration at the rear. Moreover, if a pipette containing a chemoattractant is placed to the side of a migrating leukocyte, the overall concentration of cytosolic Ca2+ first increases and then the Ca2+ gradient reorients, with the lowest concentration on the side of the cell closest to the pipette, causing the cell to turn toward the chemotactic source. After the chemoattrac-tant is removed, the cell continues to move in the direction of its newly established Ca2+ gradient (see Figure 5-47).
We have seen that many actin-binding proteins, including myosins I and II, gelsolin, a-actinin, and fimbrin, are regulated by Ca2 + . Hence the cytosolic Ca2+ gradient may regulate the sol-to-gel transitions that take place in cell movement. The low Ca2+ concentration at the front of the cell would favor the formation of actin networks by activating myosin I, inactivating actin-severing proteins, and reversing the inhibition of Ca2+-regulated actin cross-linking proteins. The high Ca2+ concentration at the rear of the cell would cause actin networks to disassemble and a sol to form by activating gelsolin or would cause cortical actin networks to contract by activating myosin II. Thus an internal gradient of Ca2+ would contribute to the turnover of actin filaments in migrating cells.
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