| We found that AKAP 4 binds AKAP 3 and two novel spermatogenic cell specific proteins , Fibrous Sheath Interacting Proteins 1 and 2 ( FSIP 1 , FSIP 2 ) . ^^^ Transcription of Akap 4 , Akap 3 , and Fsip 1 begins in early spermatid development , whereas transcription of Fsip 2 begins in late spermatocyte development . ^^^ |