Introduction

All cells have evolved regulatory mechanisms which allow them to respond to external signals and are essential for cell multiplication and survival. In mammalian cells, an array of signal transduction cascades have been described that respond to growth factors and hormones (Mooibroek & Wang, 1988; Su & Karin, 1996). More recently, highly homologous signalling cascades have been reported in the yeasts Saccharomyces cerevisiae and Schizosucchuromyces pombe, which are involved in a variety of cellular processes including mating, hyper and hype-osmotic sensing, invasive filamentous growth and cell wall integrity (Nishida & Gotoh, 1993; Roberts & Fink, 1994; Waskiewicz & Cooper, 1995; Su & Karin, 1996; Cahil, Janknecht & Nordheim,1996). The presence of such highly conserved signal transduction pathways suggests that these signal cascades may first have evolved in eukaryotic microbes and have been conserved and adapted during eukaryotic evolution (Janssens, 1987; Kincaid, 1991; Csaba, I994;Gadd, 1995; Rasmussen et al., 1996).

One of the most highly studied signal transduction pathways in mammalian cells is the phosphoinositide cycle which has been the centre of intense research since the first report that inositol 1, 4, 5-trisphosphate (Ins(1, 4, 5)P3), acts as a second messenger, mobilizing Ca2+ from intracellular stores in response to a variety of growth factors, hormones and other ligands (for reviews see Berridge & Irvine, 1984; Nishizuka, 1984;Divecha & Irvine, 1995).