WS ISH NE Can Natan Sharansky keep Israel's government EW IT J afloat and still be TRO a bridge to DE E TH American Jews? 50 NEIL RUBIN SPECIAL TO THE JEWISH NEWS NI atan Sharansky's seemingly omnipresent smile evaporates like the vapor of breath on a winter Moscow street. "Definitely these 11 months or year that I'm part of the political establishment is not the most enjoyable year in my life," he says. Many American Jews need no reminder that Mr. Sharansky, the former Soviet dissident, has a number of least enjoyable years to choose from. He says they pale in comparison with the year he has just completed as a key player in the bruising world of Israeli politics.