Proline is a very unusual amino acid, in that the side-chain is cyclized back on to the backbone amide position. This has three important consequences. First, the backbone conformation of proline itself is very restricted. The available backbone sb dihedral angles are limited to a small range around = -65 [1,2] (Figure 1). Similar restrictions do not apply to i/, which is able to populate either the a-helical region (VY -40 0) or the f-sheet region (Q approx. + 150 0). Surveys of prolines in crystal structures show that roughly 44% of prolines are in the a region and 560% are in the ,3 region [4,5]. Second, the bulkiness of the N-CH2 group places restrictions on the conformation of the residue preceding proline [6], disfavouring the a-helix conformation [4,5]. Third, because the amide proton is replaced by

The structure and function of proline-rich regions in proteins.

SPERMATOGENESIS is a complex developmental process that occurs in several phases. A large number of genes have been identified that are expressed during spermatogenesis1,2, but the biological significance of many of these is not yet known. We have used gene targeting to selectively eliminate the transcription factor CREM (cyclic AMP-responsive element modulator), which is thought to e important for mammalian spermatogenesis3–5. Male mice deficient for all CREM proteins are sterile, as their developing spermatids fail to differentiate into sperm, and postmeiotic gene expression in the testis declines dramatically. The cessation of sperm development is not accompanied by decreases in the levels of follicle-stimulating hormone or testosterone. Our findings indicate that the CREM gene is essential for spermatogenesis, and mice deficient for this transcription factor could serve as a model system for the study of idiopathic infertility in men.

Severe impairment of spermatogenesis in mice lacking the CREM gene

The transition nuclear proteins (TPs) constitute 90% of the chromatin basic proteins during the steps of spermiogenesis between histone removal and the deposition of the protamines. We first summarize the properties of the two major transition nuclear proteins, TP1 and TP2, and present concepts, based on their time of appearance in vivo and in vitro properties, regarding their roles. Distinct roles for the two TPs in histone displacement, sperm nuclear shaping, chromatin condensation, and maintenance of DNA integrity have been proposed. More definitive information on their roles in spermiogenesis has recently been obtained using mice with null mutations in the Tnp1 or Tnp2 genes for TP1 and TP2, respectively. In these mice, histone displacement and sperm nuclear shaping appear to progress quite normally. Spermatid nuclear condensation occurs, albeit in an abnormal fashion, and the mature sperm of the Tnp -null mutants are not as condensed as wild-type sperm. There is also evidence that sperm from these mutant mice contain an elevated level of DNA strand breaks. The mutant sperm showed several unexpected phenotypes, including a high incidence of configurational defects, such as heads bent back on midpieces, midpieces in hairpin configurations, coils, and clumps, other midpiece defects, reduced levels of proteolytic processing of protamine 2 during maturation, and reduced motility. The two TPs appear partly to compensate for each other as both Tnp1 - and Tnp2 -null mice were able to produce offspring, and appear to have largely overlapping functions as the two mutants had similar phenotypes.

Roles of transition nuclear proteins in spermiogenesis

Sperm from mice carrying a targeted mutation of the acrosin gene can penetrate the oocyte zona pellucida and effect fertilization.

Transition nuclear proteins (TPs), the major proteins found in chromatin of condensing spermatids, are believed to be important for histone displacement and chromatin condensation during mammalian spermatogenesis. We generated mice lacking the major TP, TP1, by targeted deletion of the Tnp1 gene in mouse embryonic stem cells. Surprisingly, testis weights and sperm production were normal in the mutant mice, and only subtle abnormalities were observed in sperm morphology. Electron microscopy revealed large rod-like structures in the chromatin of mutant step 13 spermatids, in contrast to the fine chromatin fibrils observed in wild type. Steps 12–13 spermatid nuclei from the testis of Tnp1-null mice contained, in place of TP1, elevated levels of TP2 and some protamine 2 (P2) precursor. Most of the precursor was processed to mature P2, but high levels of incompletely processed forms remained in epididymal spermatozoa. Sperm motility was reduced severely, and ≈60% of Tnp1-null males were infertile. We concluded that TP1 is not essential for histone displacement or chromatin condensation. The absence of TP1 may partially be compensated for by TP2 and P2 precursor, but this dysregulation of nucleoprotein replacement results in an abnormal pattern of chromatin condensation and in reduced fertility.

https://www.pnas.org/content/pnas/97/9/4683.full.pdf

Abnormal spermatogenesis and reduced fertility in transition nuclear protein 1-deficient mice

Complementary DNA clones for the boar preproacrosin have been isolated from a randomly primed testis cDNA library in λgt10 and from an oligo(dT)-primed testis cDNA in λt11. The nucleotide sequence of the 1418-bp cDNA insert includes a 46-bp 5′-untranslated region, an open reading frame of 1248 bp corresponding to 416 amino acids (45.59 kDa) and a 121-bp 3′-untranslated region. The deduced amino acid sequence includes the active-site residues histidine, asparagine and serine of the catalytic triad of the serine proteinase super-family and is colinear with that determined by amino acid sequencing of the boar acrosin light chain and of a small region of the NH2-terminal sequence of the heavy chain. The preproacrosin cDNA contains at the 3′ end a 381-bp sequence which codes for an amino acid sequence not yet found in any other serine proteinase. This amino acid sequence is rich in proline (42 out of 127 amino acids) and is suggested to be involved in the recognition and binding of the spermatozoa to the zona pellucida of the ovum. The mRNA for preproacrosin is synthesizd as an approximately 1.6-kb-long molecule only in the postmeiotic stages of boar and bull spermatogenesis.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1989.tb14864.x

Molecular cloning of preproacrosin and analysis of its expression pattern in spermatogenesis

Mouse preproacrosin: cDNA sequence, primary structure and postmeiotic expression in spermatogenesis

Nucleotide sequences and expression of cDNA clones for boar and bull transition protein 1 and its evolutionary conservation in mammals

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Molecular cloning of preproacrosin and analysis of its expression pattern in spermatogenesis

Mouse preproacrosin: cDNA sequence, primary structure and postmeiotic expression in spermatogenesis

Nucleotide sequences and expression of cDNA clones for boar and bull transition protein 1 and its evolutionary conservation in mammals