The recruitment of the small GTPase Arf6 and ARNO from cytosol to endosomal membranes is driven by V-ATPase-dependent intra-endosomal acidification. The molecular mechanism that mediates this pH-sensitive recruitment and its role are unknown. Here, we demonstrate that Arf6 interacts with the c-subunit, and ARNO with the a2-isoform of V-ATPase. The a2-isoform is targeted to early endosomes, interacts with ARNO in an intra-endosomal acidification-dependent manner, and disruption of this interaction results in reversible inhibition of endocytosis. Inhibition of endosomal acidification abrogates protein trafficking between early and late endosomal compartments. These data demonstrate the crucial role of early endosomal acidification and V-ATPase/ARNO/Arf6 interactions in the regulation of the endocytic degradative pathway. They also indicate that V-ATPase could modulate membrane trafficking by recruiting and interacting with ARNO and Arf6; characteristics that are consistent with the role of V-ATPase as an essential component of the endosomal pH-sensing machinery.

A systematic mutagenesis of the SV40 enhancer indicates that it spans -100 bp and is composed of at least two distinct DNA domains which exhibit very little enhancing activity on their own. Their association results in a 400-fold enhancement of transcription, virtually irrespective of their relative orientation and, to some extent, of the distance between them. Enhancer activity can also be generated by duplication of either domain. We show also that the activity of each domain is due to the presence of several specific sequence motifs. These motifs are found assorted in different combinations in other viral and cellular enhancers. Key words: transcription/site-directed mutagenesis/promoter/ RNA polymerase B(ll)/simian virus 40 1983;Lusky et al., 1983;Hearing and Shenk, 1983;Veldman et al., 1985). However, such enhancer consensus sequences occur in DNA segments without enhancer properties, and enhancer activity can be generated by duplicating DNA sequences without enhancer activity on their own (Weber et al., 1984;Swimmer and Shenk, 1984), thus questioning the real functional significance of these consensus sequences.Therefore, we decided to determine, at the nucleotide level, the DNA sequences essential for the activity of the prototype SV40 enhancer. Systematic deletions and point mutations have been constructed throughout the enhancer region and their effect on SV40 early transcription investigated in vivo, using a transient expression assay in HeLa cells. We show here that the SV40 enhancer encompasses a large DNA segment of -100 nucleotides containing the 72-bp sequence, but also extending further upstream. Furthermore, the present study reveals that the enhancer is composed of at least two distinct DNA domains which exhibit very little enhancing activity on their own. However, their association results in a dramatic 400-fold enhancement of transcription, virtually irrespective of their relative orientation and, to some extent, of the distance between them. Enhancer activity can also be generated by duplication of either domain. In addition, we show that the activity of each domain is due to the presence of several specific sequence motifs. Various assortments of these motifs are found in other viral and cellular enhancers, suggesting that enhancers are mosaics of a limited number of basic evolutionary related sequence motifs.

SummaryAfter internalization into macrophages non-pathogenic mycobacteria are killed within phagosomes. Pathogenic mycobacteria can block phagosome maturation and grow inside phagosomes but under some conditions can also be killed by macrophages. Killing mechanisms are poorly understood, although phagolysosome fusion and nitric oxide (NO) production are implicated. We initiated a systematic analysis addressing how macrophages kill 'non-pathogenic' Mycobacterium smegmatis . This system was dynamic, involving periods of initial killing, then bacterial multiplication, followed by two additional killing stages. NO synthesis represented the earliest killing factor but its synthesis stopped during the first killing period. Phagosome actin assembly and fusion with late endocytic organelles coincided with the first and last killing phase, while recycling of phagosome content and membrane coincided with bacterial growth. Phagosome acidification and acquisition of the vacuolar (V) ATPase followed a different pattern coincident with later killing phases. Moreover, V-ATPase localized to vesicles distinct from classical late endosomes and lysosomes. Map kinase p38 is a crucial regulator of all processes investigated, except NO synthesis, that facilitated the host for some functions while being usurped by live bacteria for others. A mathematical model argues that periodic high and low cellular killing activity is more effective than is a continuous process.

Stimulation of in vitro transcription by the simian virus 40 enhancer involves a rapid and stable binding of a trans-acting factor with both the 5'- and 3'-domains of the enhancer sequence. The enhancer factor, which differs from other types of transcriptional factors, can interact with other enhancer elements.