Castanea Partners said Monday it has acquired a minority stake in Proenza Schouler. Financial terms weren’t announced. Proenza Schouler is a New York-based luxury fashion brand. Shirley Cook, Proenza Schouler’s CEO, is stepping down as part of the deal. Ron Frasch, former President and Chief Merchandising Officer at Saks Fifth Avenue and currently an operating partner at Castanea, is assuming the role as Interim CEO.