Weil, Gotshal & Manges partner Kenneth Steinthal is at the center of the latest copyright battle to pit traditional media against emerging and established companies involved in online content distribution.

He’s defending YouTube, the wildly popular video streaming Web site, against the kind of copyright infringement complaints that could wipe out the San Mateo, Calif., startup. The case weighs whether the 35-employee company has a responsibility to police its millions of postings against demands of an irate copyright holder.