54 Actin dynamics in viva Matthew D Welch*, Aneil Mallavarapuf, Jody Rosenblattf and Timothy J MitchisonS zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Ac tin d yna m ic s in la m e llip o d ia a re d rive n b y c o ntinuo us c yc le s o f a c tin p o lym e riza tio n, re tro g ra de flo w , a nd d e p o lym e riza tio n. In the past ye a r, a d va nc e s have been made in identifying sig na ling pathways tha t re g ula te a c tin-fila m e nt uncapping a nd p o lym e riza tio n, in d e te rm ining the ro le o f myo sin m o to r p ro te ins in re tro g ra d e flo w , and in evaluating the ro le o f se ve ring p ro te ins in a c tin d e p o lym e riza tio n. Bo th Liste ria monocytogenes and Saccharomyces cerevisiae have emerged a s p o w e rful model o rg a nism s fo r stud ying a c tin d yna m ic s in c e lls. Ad d re sse s **De p a rtm e nt o f C e llula r a nd Mo le c ula r Pha rm a c o lo g y, Unive rsity of C a lifo rnia , Sa n Fra nc isc o , C A 94143.0450, USA *e -m a il: w e lc h@ c g l.uc sf.e d u tDe p a rtm e nt o f Bio c he mistry a nd Bio p hysic s, Unive rsity o f C a lifo rnia , Sa n Fra nc isc o , C A 94143.0450, USA C urre nt O p inio n in C e ll Bio lo g y 1997, 9:54-61 Ele c tro nic ide ntifie r: 0955-0674-009-00054 0 C urre nt Bio lo g y Ltd ISSN 0955-0674 Ab b re via tio ns ADF a c tin-d e p o lym e rizing fa c to r Arp a c tin-re la te d p ro te in BDM butane dione monoxime C ALI c hro m o p ho re -a ssiste d la se r ina c tiva tio n Introduction The leading edge of a motile cell is composed of thin protrusions of membrane which continuously extend and retract, mediating the initial stage of cell movement and determining the direction of advance. The underlying cytoskeleton of a leading edge is composed of actin- filament bundles (in filopodia) or meshworks (in lamellipo- dia) oriented primarily with their ‘barbed’ (fast-growing) ends towards the membrane (Fig. 1). In all cell types studied so far, leading-edge actin filaments undergo a continuous cycle of assembly at the inner-membrane surface, transport away from the membrane in a process termed ‘centripetal transport’ or ‘actin retrograde flux or flow’, and, finally, disassembly or depolymerization [l]. Here, we review recent advances in our understanding of leading-edge actin dynamics. We will address the regulation of actin polymerization, transport and depoly- merization in the leading edge, and how these processes relate to the generation of motile force. We will highlight outstanding questions in the field, and emerging systems for studying these problems. We have restricted our discussion to leading-edge dynamics because this area has seen most progress recently. Actin dynamics in the cell body are less well understood, though they may be equally Figure 1 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON Nut e xe Pla sm a m e m b ra ne 0 1997 Current Ooinion in Cell Biolocn Ac tin d yna m ic s in the le a d ing e d g e o f m o tile c e lls. Le a d ing -e d g e a c tin fila m e nts und e rg o c o ntinuo us p o lym e riza tio n o nto fila m e nt b a rb e d (fa st-g ro w ing ) e nd s (sho w n a t rig ht) a t the inne r surfa c e o f the p la sm a m e m b ra ne . Ne w ly p o lym e rize d fila m e nts a re tra nsp o rte d to w a rd s the c e ll inte rio r (to w a rd s the le ft) b y a p ro c e ss c a lle d re tro g ra d e flo w . Fina lly, fila m e nts a re d e p o lym e rize d a nd sub units a re re c yc le d (b y e xc ha ng ing ADP fo r ATP) fo r a ne w ro und o f p o lym e riza tio n. or more important for understanding cell locomotion as a whole [Z]. Actin polymerization It has been known for some time that actin monomers are rapidly incorporated into filaments at the leading edge of cells, and it is widely accepted that polymerization of new filaments and membrane protrusion are tightly coupled. Outstanding questions include the role of polymerization in generating force for protrusion, and how polymerization is regulated. The issue of regulation has seen considerable progress over the past year, in part because it ties in to signal transduction through pathways that also regulate cell proliferation (see Tapon and Hall, this issue, pp 86-92). The fuel for actin polymerization at the leading edge is the large cellular pool of unpolymerized actin, which is maintained at concentrations well above those needed for actin polymerization. To maintain this pool, cells employ monomer-sequestering proteins (i.e. profilin and thymosin 84) which control polymerization through monomer avail- ability [3] and barbed end capping proteins (i.e. Cap2 and gelsolin) which regulate polymerization onto pre-existing filament ends [4,5]. The relative importance of these mechanisms may vary between cells. For example, genetic evidence suggests that profilin promotes actin polymerization in some cells and inhibits it in others [3]. Similarly, Cap2 is required to prevent filament elongation in zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP Dictyostehwn where the concentration of unpolymerized actin is high [6”] and to protect filaments