We have examined the differential binding of Hck and Fyn to HIV‐1 Nef to elucidate the structural basis of SH3 binding affinity and specificity. Full‐length Nef bound to Hck SH3 with the highest affinity reported for an SH3‐mediated interaction (KD 250 nM). In contrast to Hck, affinity of the highly homologous Fyn SH3 for Nef was too weak (KD > 20 microM) to be accurately determined. We show that this distinct specificity lies in a variable loop, the ‘RT loop’, positioned close to conserved SH3 residues implicated in the binding of proline‐rich (PxxP) motifs. A mutant Fyn SH3 with a single amino acid substitution (R96I) in its RT loop had an affinity (KD 380 nM) for Nef comparable with that of Hck SH3. Based on additional mutagenesis studies we propose that the selective recognition of Nef by Hck SH3 is determined by hydrophobic interactions involving an isoleucine residue in its RT loop. Although Nef contains a PxxP motif which is necessary for the interaction with Hck SH3, high affinity binding was only observed for intact Nef protein. The binding of a peptide containing the Nef PxxP motif showed > 300‐fold weaker affinity for Hck SH3 than full‐length Nef.

Background: Mitoxantrone is used for aggressive multiple sclerosis (MS), but concerns about

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The mammary cancer cell line CAMA-1 synchronized at the G1/S boundary by thymidine block or at the G1/M boundary by nocodazole was used to evaluate 1) the sensitivity of a specific cell cycle phase or phases to 17 beta-estradiol (E2), 2) the effect of E2 on cell cycle kinetics, and 3) the resultant E2 effect on cell proliferation. In synchronized G1/S cells, E2-induced 3H-thymidine uptake, which indicated a newly formed S population, was observed only when E2 was added during, but not after, thymidine synchronization. Synchronized G2/M cells, enriched by Percoll gradient centrifugation to approximately 90% mitotic cells, responded to E2 added immediately following selection; the total E2-treated population traversed the cycle faster and reached S phase approximately 4 hr earlier than cells not exposed to E2. When E2 was added during the last hour of synchronization (ie, at late G2 or G2/M), or for 1 hr during mitotic cell enrichment, a mixed response occurred: a small portion had an accelerated G1 exit, while the majority of cells behaved the same as controls not incubated with E2. When E2 addition was delayed until 2 hr, 7 hr, or 12 hr following cell selection, to allow many early G1 phase cells to miss E2 exposure, the response to E2 was again mixed. When E2 was added during the 16 hr of nocodazole synchronization, when cells were largely at S or possibly at early G2, it inhibited entry into S phase. The E2-induced increase or decrease of S phase cells in the nocodazole experiments also showed corresponding changes in mitotic index and cell number. These results showed that the early G1 phase and possibly the G2/M phase are sensitive to E2 stimulation, late G1, G1/S, or G2 are refractory; the E2 stimualtion of cell proliferation is due primarily to an increased proportion of G1 cells that traverse the cell cycle and a shortened G1 period, E2 does not facilitate faster cell division; and estrogen-induced cell proliferation or G1/S transition occurs only when very early G1 phase cells are exposed to estrogen. These results are consistent with the constant transition probability hypothesis, that is, E2 alters the probability of cells entering into DNA synthesis without significantly affecting the duration of other cell cycle phases. Results from this study provide new information for further studies aimed at elucidating E2-modulated G1 events related to tumor growth.

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Both serum and estrogen affect cell proliferation in CAMA-1 cells. Their effects on cell cycle kinetics are being investigated with partially synchronized cells following 48 hours of serum deprivation. By comparing the growth kinetics of synchronized (serum-deprived) cells with asynchronized (normal-fed) cells, we observed that there was a delay of cell proliferation for approximately 25 hr for synchronized cells and that estrogen only induced cell proliferation in serum-supplemented culture. A serum protein (RPF), precipitated by ammonium sulfate, dialyzed in 3,500 daltons cut off membrane and reconstituted in culture medium, was shown to stimulate cell proliferation in a dose-related manner. In the absence of RPF, estrogen had no effect on cell growth and S-phase formation in serum-free medium, but significantly induced cell growth in the presence of RPF. Experiments conducted with a synchronized cell population showed that estrogen increased the proportion of G1 phase cells to enter S phase, shortened the G1 phase duration, and ultimately increased the proportion of mitotic cells per cycle. Collectively, these results show that (a) estrogen effects of cell proliferation in vitro requires a serum component, a "G1/S-promoting factor" and (b) estrogen-induced tumor growth is a result of an accelerated rate of G1/S transition and an increased number of dividing cells per cycle. The knowledge of the interaction of estrogen with the G1/S promoting factor on cell growth is of both fundamental and therapeutic importance.

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