Cuttlefish spermiogenesis is characterized by a double nuclear protein transition: histones -> spermatid-specific proteins (T1/T2) -> protamines (SP1/SP2). The protamines compact sperm DNA into a highly condensed, stable and inactive complex. Testis. Spermiogenesis. Phosphorylation occurs at different degrees. The triphosphorylated form may be predominant in T2. SP2 appears to be phosphorylated in elongated spermatids, but dephosphorylated in mature sperm cells.