Here we describe a protein product of the human septin H5/PNUTL2/CDCrel2b gene, which we call ARTS (for apoptosis-related protein in the TGF-beta signalling pathway). ARTS is expressed in many cells and acts to enhance cell death induced by TGF-beta or, to a lesser extent, by other apoptotic agents. Unlike related septin gene products, ARTS is localized to mitochondria and translocates to the nucleus when apoptosis occurs. Mutation of the P-loop of ARTS abrogates its competence to activate caspase 3 and to induce apoptosis. Taken together, these observations expand the functional attributes of septins previously described as having roles in cytokinesis and cellular morphogenesis.

Accumulated indirect evidence suggests nerve growth-promoting activities for acetylcholinesterase (AChE). To determine unequivocally whether such activities exist, whether they are related to the capacities of this enzyme to hydrolyze acetylcholine and enhance synapse development, and whether they are associated with alternative splicing variants of AChEmRNA, we used four recombinant human AChEDNA vectors. When Xenopus laevis embryos were injected with a vector expressing the synapse-characteristic human AChE-E6, which contains the exon 6-encoded C terminus, cultured spinal neurons expressing this enzyme grew threefold faster than co-cultured control neurons. Similar enhancement occurred in neurons expressing an insertion-inactivated human AChE-E6-IN protein, containing the same C terminus, and displaying indistinguishable immunochemical and electrophoretic migration properties from AChE-E6, but incapable of hydrolyzing acetylcholine. In contrast, the nonsynaptic secretory human AChE-I4, which contains the pseudointron 4-derived C terminus, did not affect neurite growth. Moreover, no growth promotion occurred in neurons expressing the catalytically active C-terminally truncated human AChE-E4, demonstrating a dominant role for the E6-derived C terminus in neurite extension. Also, AChE-E6 was the only active enzyme variant to be associated with Xenopus membranes. However, postsynaptic length measurements demonstrated that both AChE-E6 and AChE-E4 enhanced the development of neuromuscular junctions in vivo, unlike the catalytically inert AChE-E6-IN and the nonsynaptic AChE-I4. These findings demonstrate an evolutionarily conserved synaptogenic activity for AChE that depends on its hydrolytic capacity but not on its membrane association. Moreover, this synaptogenic effect differs from the growth-promoting activity of AChE, which is unrelated to its hydrolytic capacity yet depends on its exon 6-mediated membrane association.

In adrenal glomerulosa cells, angiotensin II (Ang II) and potassium stimulate aldosterone synthesis through activation of the calcium messenger system. The ratelimiting step in steroidogenesis is the transfer of cholesterol to the inner mitochondrial membrane. This transfer is believed to depend upon the presence of the steroidogenic acute regulatory (StAR) protein. (Ang II) 1 and K ϩ act as regulators of aldosterone synthesis and secretion in adrenal glomerulosa cells. The crucial role of the Ca 2ϩ messenger in the acute regulation of aldosterone production is firmly established (1-5). Indeed, the steroidogenic response of isolated adrenal cells to Ang II and K ϩ is highly dependent upon extracellular Ca 2ϩ concentration (6) and can be blocked by inhibitors of Ca 2ϩ influx across the plasma membrane (4). Moreover, calmodulin antagonists have been shown to inhibit Ang IIstimulated aldosterone production in zona glomerulosa cells (7).Traditionally, aldosterone biosynthesis is functionally divided into three consecutive phases. (i) In the early mitochondrial steps, cholesterol is transported from intracellular lipid droplets into the outer mitochondrial membrane (OM) and then to the inner mitochondrial membrane (IM). The latter step represents the rate-limiting process in all steroidogenic pathways (8) and is followed by the conversion of cholesterol to pregnenolone by the cytochrome P450 scc enzyme. (ii) The intermediate steps take place on the endoplasmic reticulum and involve the conversion of pregnenolone to progesterone by 3␤-hydroxysteroid dehydrogenase isomerase and then to 11-deoxycorticosterone. (iii) The late steroidogenic steps are localized back in the mitochondria and include the formation of corticosterone and its conversion to aldosterone by cytochrome P450 11␤ .The regulation of intramitochondrial cholesterol transfer by cAMP-dependent mechanisms has been extensively studied (9). While the transport of cholesterol from lipid droplets to the outer mitochondrial membrane was found not to be affected by inhibitors of protein synthesis in ACTH-stimulated adrenal

We conclude that upsetting cholinergic balance may by itself cause progressive memory decline in mammals, suggesting that congenital and/or acquired changes in this vulnerable balance may contribute to the physiopathology of Alzheimer's disease.

Steroidogenic acute regulatory protein (StAR) is a nuclear encoded mitochondrial protein that enhances steroid synthesis by facilitating the transfer of cholesterol to the inner membranes of mitochondria in hormonally regulated steroidogenic cells. It is currently assumed that StAR activity commences before or during StAR import into the mitochondrial matrix. The present study was designed to demonstrate that, once imported and becoming physiologically irrelevant, exhaustive accumulation of StAR must be limited by a rapid degradation of the protein to prevent potential damage to the organelles. The use of uncouplers and manipulation of the interior mitochondrial pH in hormone-induced ovarian granulosa cells and StAR-expressing COS cells suggests that StAR degradation is biphasic and involves two classes of proteases. During phase I, which normally lasts for the first approximately 2 h following import, StAR is rapidly degraded by a protease, or proteases, that can be arrested by a nonclassical action of proteasome inhibitors such as MG132. StAR molecules that evade phase I are subjected to a second class of protease(s), which is slower and MG132 resistant. A third proteolytic entity was revealed in studies with C-28 StAR, a loss-of-function mutant of StAR. Upon initiation of its import, C-28 StAR dissipates the inner membrane potential and causes swelling of the mitochondria. Degradation of C-28 StAR, probably by an intermembrane space protease, is extremely rapid and MG132 insensitive. Collectively, this study defines StAR as the first naturally occurring mitochondrial protein that can serve as a substrate to probe multiple proteolytic activities in mammalian mitochondria.