Among the multiplicity of roles suggested for proteins of the annexin family, those implicating these proteins in regulated exocytosis remain among the most convincing. Studies in this area of annexin biology have focused on annexin II, which because of its unusually low Ca(2+)-requirement for phospholipid-binding has many of the requisite properties of a membrane fusogenic Ca2+ sensor. Other annexins are also good candidates for exocytotic mediators, especially annexins I and VII, which have strong vesicle-aggregating activities. In contrast, annexin VI appears to block vesicle aggregation, perhaps acting as a negative regulator of exocytosis. In this review, we consider the evidence for and against annexins having functions in the secretory pathway.

To gain insight into the molecular basis of annexin gene expression we have analysed the annexin I and VI gene promoters. A previously described 881 bp sequence immediately upstream of the annexin I transcription start site and a similar size fragment proximal to the annexin VI transcription start site both drove expression of the luciferase reporter gene in fibroblasts and epithelial cells. Neither promoter displayed any sensitivity to dexamethasone, suggesting that the putative glucocorticoid response element in the annexin I promoter is non-functional. Consistent with this, endogenous annexin I gene expression was unaffected by dexamethasone at the mRNA and protein levels in A431 cells. A series of 5' deletions of the two promoters were examined to define the minimal active sequences. For annexin I this corresponded to a sequence approx. 150 bp upstream of the transcription start site that included CAAT and TATA boxes. Unexpectedly, the annexin VI promoter, which also contains CAAT and TATA boxes, was fully active in the absence of these elements, a 53 bp sequence between these boxes and the transcription start site having maximal activity. Electrophoretic mobility-shift assays with nuclear extracts from A431 and HeLa cells with probes corresponding to this region revealed an SP1-binding site. These results show that the annexin I and VI genes have individual modes of transcriptional regulation and that if either annexin I or annexin VI has an anti-inflammatory role, then this is in the absence of steroid-induced gene expression.

Mammalian genomes contain numerous fragments of DNA that are derived from inactivated transposable elements. The accumulation and persistence of these elements is generally attributed to transposase activity rather than through possession or acquisition of a function of value to the host genome. Here we describe such a repetitive element, named ALF (forannexin VILINE-2fragment), comprising 130 bp of DNA derived from a LINE-2 sequence, which functions as a potent T-cell-specific silencer. The expansion of the DNA database arising as a result of the human genome sequencing project enabled us to identify ALF in, or close to, several well characterized genes including those for annexin VI, interleukin-4 and protein kinase C-beta. A systematic analysis of the entire LINE-2 sequence revealed that ALF, and not other regions of the LINE-2 sequence, was especially highly represented in the human genome. Acquisition of a function by this repetitive element may explain its abundance. These data show that a conserved fragment of an interspersed nuclear element has the potential to modulate gene expression, a discovery that has broad implications for the way in which we view so-called 'junk' DNA and our understanding of eukaryotic gene regulation.