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Most newly synthesized peroxisomal matrix proteins are targeted to the organelle by Pex5p, the peroxisomal cycling receptor. According to current models of peroxisomal biogenesis, Pex5p interacts with cargo proteins in the cytosol and transports them to the peroxisomal membrane. After delivering the passenger protein into the peroxisomal matrix, Pex5p returns to the cytosol to catalyze additional rounds of transportation. Obviously, such cyclic pathway must require energy, and indeed, data confirming this need are already available. However, the exact step(s) of this cycle where energy input is necessary remains unclear. Here, we present data suggesting that insertion of Pex5p into the peroxisomal membrane does not require ATP hydrolysis. This observation raises the possibility that at the peroxisomal membrane ATP is needed predominantly (if not exclusively) downstream of the protein translocation step to reset the Pex5p-mediated transport system.Peroxisomal matrix proteins are synthesized on cytosolic ribosomes and post-translationally imported into the organelle. The vast majority of these proteins possess the so-called peroxisomal targeting sequence 1 (PTS1), 1 a C-terminal tripeptide complying to the consensus sequence S/A/C-K/R/H-L/M (1-4). These PTS1-containing proteins are specifically targeted to the organelle matrix by Pex5p, the PTS1 receptor (5-9).According to current models of peroxisomal biogenesis, the Pex5p-mediated process of protein import can be divided into four steps. In the first step, newly synthesized PTS1-containing proteins interact with Pex5p in the cytosol. This protein-protein interaction involves the PTS1 signal on one side and the tetratricopeptide repeats domain of Pex5p on the other. The Pex5p-cargo protein complex is then recognized by the so-called docking machinery present in the peroxisomal membrane. Somewhere after this event, the PTS1-containing protein is released into the peroxisomal matrix. Finally, Pex5p is recycled back to the cytosol to catalyze additional rounds of transportation (reviewed in Refs. 10 -12).One obvious property of such cyclic mechanism is that it needs some form of energy input to function, and indeed, basically all the studies addressing this issue are unanimous in this respect: protein import into the peroxisomal matrix requires hydrolysis of ATP (13-21). However, the precise step(s) of this import pathway where energy input is necessary has not been firmly established. For instance, it is generally accepted that the step of protein translocation across the peroxisomal membrane requires ATP hydrolysis. Such conclusion derives from the fact that ATP depletion or the inclusion of non-hydrolysable ATP analogues in the several experimental systems used result in an inhibition of the peroxisomal import process. However, the possibility that recycling of Pex5p back to the cytosol is an ATP-dependent event and the rate-limiting step in all this process was never considered. In this scenario, inhibition of peroxisomal protein import by lack of ATP would resu...

It is now generally accepted that Pex5p, the receptor for most peroxisomal matrix proteins, cycles between the cytosol and the peroxisomal compartment. According to current models of peroxisomal biogenesis, this intracellular trafficking of Pex5p is coupled to the transport of newly synthesized peroxisomal proteins into the organelle matrix. However, direct evidence supporting this hypothesis was never provided. Here, using an in vitro peroxisomal import system, we show that insertion of Pex5p into the peroxisomal membrane requires the presence of cargo proteins. Strikingly the peroxisomal docking/translocation machinery is also able to catalyze the membrane insertion of a Pex5p truncated molecule lacking any known cargo-binding domain. These results suggest that the cytosol/peroxisomal cycle in which Pex5p is involved is directly or indirectly regulated by Pex5p itself and not by the peroxisomal docking/translocation machinery.

According to current models of peroxisomal biogenesis, Pex5p cycles between the cytosol and the peroxisome transporting newly synthesized proteins to the organelle matrix. However, little is known regarding the mechanism of this pathway. Here, we show that Pex5p enters and exits the peroxisomal compartment in a process that requires ATP. Insertion of Pex5p into the peroxisomal membrane is blocked by anti-Pex14p IgGs. At the peroxisomal level, two Pex14p-associated populations of Pex5p could be resolved, stage 2 and stage 3 Pex5p, both exposing the majority of their masses into the organelle lumen. Stage 3 Pex5p can be easily detected only under ATP-limiting conditions; in the presence of ATP it leaves the peroxisomal compartment rapidly. Our data suggest that translocation of PTS1-containing proteins across the peroxisomal membrane occurs concomitantly with formation of the Pex5p-Pex14p membrane complex and that this is probably the site from which Pex5p leaves the peroxisomal compartment.

Normal pubertal development in humans involves two distinct processes: maturation of adrenal androgen secretion (adrenarche) and activation of the hypothalamic-pituitary-gonadal axis (gonadarche). One factor thought to contribute to the adrenarche in man is increased adrenal 17-hydroxylase (CYP17) activity. In the rat, there is evidence for adrenal involvement in the initiation of puberty, but the adrenal glands of this species are generally thought to express CYP17 only very poorly at best. To further examine the nature of postnatal adrenal development in rat, plasma samples and adrenal tissues were taken from animals aged 2-90 days, circulating adrenal steroids assayed, and adrenal zones assessed quantitatively. A relative increase in zona reticularis, and peaks of circulating cortisol, androstenedione, and 17-OH-progesterone were observed around postnatal days 16-20, clearly before the development of the gonads, which begins at 30-35 days. Quantitative reverse transcriptase PCR confirmed a peak in mRNA coding for CYP17 in adrenal tissue from rats of similar age. The results suggest that the rat adrenal has the capacity to secrete steroids arising from 17-hydroxylation, and that this may contribute to a process similar to human adrenarche.

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Lung cancer (LC) is a serious public health problem responsible for the majority of cancer deaths and comorbidities in developed countries. Tobacco smoking is considered the main risk factor for LC; however, only a few smokers will be affected by this cancer. Current screening methods are focused on identifying the early stages of this malignancy. Thus, new data concerning the roles of microRNA alterations in inflammation, epithelial-mesenchymal transition and lung disease have increased hope about LC pathogenesis, diagnosis, treatment and prognosis. MicroRNA mechanisms include angiogenesis promotion, cell cycle regulation by modulating cellular proliferation and apoptosis, and migration and invasion inhibition. In this context, this manuscript reviews the current information about many important microRNAs as they relate to the initiation and progression of LC.

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