Krebs medal review: PtdIns3P in tumour suppression of phosphorylated derivatives of phosphatidylinositol (PtdIns), collectively known as phosphoinositides (PIs), are ideally suited for this task [1]. The PIs are gener- ated and metabolized through the activities of a num- ber of substrate-speciﬁc PI kinases and phosphatases, the dysfunctions of which are associated with various human diseases [2]. Of special interest in cancer research are two PI 3-kinases (i.e. kinases that phos- phorylate the inositol headgroup in the 3-position) that have opposing roles in tumourigenesis. Class I PI 3-kinase (PI3K-I), on the one hand, is a well-known tumour promoting enzyme (or to be more precise, a small group of related enzymes) whose hyperactivity is strongly associated with carcinogenesis in humans [3]. Consistent with this, PTEN, a phosphatase that essen- tially reverses the 3-phosphorylation mediated by PI3K-I, is an important tumour suppressor [4]. The catalytic product of PI3K-I, PtdIns(3,4,5)P3, recruits several proteins involved in cell signalling to the plasma membrane, the most important one being the protein kinase AKT, which orchestrates various signal- ling cascades that promote cell growth and survival. On the other hand, class III PI 3-kinase (PI3K-III) is considered to be a tumour suppressor based on the ﬁndings that three of its accessory subunits, Beclin 1, UVRAG and BIF-1, have been independently identi- ﬁed as tumour suppressors whose partial or complete inactivation causes the increased occurrence of sponta- neous tumours in mice and (probably) humans [5–7]. Recently identiﬁed molecular and cellular mechanisms that may serve to explain the tumour suppressor func- tions of PI3K-III are the topic of the present review. PI3K-III: a conserved lipid kinase complex The catalytic subunit of PI3K-III was ﬁrst identiﬁed as Vps34, an enzyme that mediates vacuolar protein sort- ing in the yeast Saccharomyces cerevisiae [8]. By con- trast to PI3K-I, PI3K-III is conserved between yeast, plants and humans, and the human homologue of Vps34 is referred to as hVps34, PIK3C3 or VPS34. In the present review, the latter term is used. Subsequent work soon revealed that Vps34 is associated with a regulatory subunit, Vps15, a putative protein kinase [9]. More recently, Vps34 was found to engage in two functionally distinct complexes in yeast. One complex, consisting of Vps34, Vps15, Vps30 and Vps38, regu- lates endosomal sorting, whereas another complex, in which Vps38 is replaced by Atg14, is required for autophagy [10]. The two PI3K-III complexes in yeast have their human counterparts: one consisting of VPS34, VPS15, H. Stenmark the Vps30 homologue Beclin 1 and the Vps38 homo- logue UVRAG, and the other containing ATG14 (also called hAtg14, Atg14L or Barkor) instead of UVRAG [11–14]. Thus, we can consider VPS34-VPS15-Beclin 1 as a core complex onto which the accessory subunits UVRAG and ATG14 can assemble in a competitive manner (Fig. 1A). In addition, the UVRAG containing complex can associate with Rubicon, a protein that negatively regulates the function of this complex in endosomal and autophagic trafﬁcking [11,12], and with BIF-1 (also known as endophilin B1 or SH3GLB1), identiﬁed as a positive regulator of autophagy [5] (Fig. 1B). Although Rubicon can be isolated as a major constituent of UVRAG-containing PI3K-III complexes, this is not the case with BIF-1, which appears to be more transiently associated [12,15]. Nev- ertheless, the fact that BIF-1 contains an amino-termi- nal BAR domain predicted to have membrane-bending ability, as well as the ﬁnding that knockdown of BIF-1 inhibits the catalytic activity of PI3K-III, suggests that this protein is an important accessory subunit of mam- malian PI3K-III [5]. BAR domain proteins have been found to associate with membranes of high curvature [16], and it is tempting to speculate that BIF-1 might direct PI3K-III activity to such membranes. The recently solved crystal structure of Drosophila melanogaster Vps34 revealed that this PI3K has a considerably smaller ATP-binding pocket than class I PI3Ks [17]. This explains why several inhibitors of class I PI3Ks fail to inhibit class III PI3K (i.e. they are too bulky to ﬁt into the ATP-binding pocket). More importantly, the structural insight offers a rationale for designing speciﬁc PI3K-III inhibitors in the future. Even though considerable knowledge has been gained about the biochemical composition of PI3K-III, we still know little about the regulation of its catalytic activity. The catalytic activity appears to be stimulated by BIF-1, whereas Rubicon inhibits the overall func- tion of PI3K-III [5,11,12]. However, the exact mecha- nisms of these regulations remain unsolved. An important clue to the regulation of VPS34 has emerged recently with the ﬁnding that VPS34 is phosphorylated on threonine 159 by the cyclin-dependent kinase Ckd1 during mitosis, and that this causes its dissociation from Beclin 1 and an inhibition of autophagy [18]. Recognition of PtdIns3P by FYVE and phox homology (PX) domains A breakthrough in our understanding of how PI3K-III and its catalytic product control cellular functions came with the identiﬁcation of the FYVE (conserved in Fab1, YOTB, Vac1 and EEA1) domain, and the 4838 FEBS Journal 277 (2010) 4837–4848 ª 2010 The Author Journal compilation ª 2010 FEBS