Spermiogenesis is a postmeiotic process that drives development of round 
spermatids into fully elongated spermatozoa. Spermatid elongation is largely 
controlled post-transcriptionally after global silencing of mRNA synthesis from 
the haploid genome. Here, rats that differentially express EGFP from a 
lentiviral transgene during early and late steps of spermiogenesis were used to 
flow sort fractions of round and elongating spermatids. Mass-spectral analysis 
of 2D gel protein spots enriched >3-fold in each fraction revealed a 
heterogeneous RNA binding proteome (hnRNPA2/b1, hnRNPA3, hnRPDL, hnRNPK, hnRNPL, 
hnRNPM, PABPC1, PABPC4, PCBP1, PCBP3, PTBP2, PSIP1, RGSL1, RUVBL2, SARNP2, 
TDRD6, TDRD7) abundantly expressed in round spermatids prior to their 
elongation. Notably, each protein within this ontology cluster regulates 
alternative splicing, sub-cellular transport, degradation and/or translational 
repression of mRNAs. In contrast, elongating spermatid fractions were enriched 
with glycolytic enzymes, redox enzymes and protein synthesis factors. 
Retrogene-encoded proteins were over-represented among the most abundant 
elongating spermatid factors identified. Consistent with these biochemical 
activities, plus corresponding histological profiles, the identified RNA 
processing factors are predicted to collectively drive post-transcriptional 
expression of an alternative exome that fuels finishing steps of sperm 
maturation and fitness.