A new structural model of the acid-labile subunit: pathogenetic mechanisms of short stature-causing mutations

Abstract

The acid-labile subunit (ALS) is the main regulator of IGF1 and IGF2 bioavailability. ALS deficiency caused by mutations in the ALS (IGFALS) gene often results in mild short stature in adulthood. Little is known about the ALS structure-function relationship. A structural model built in 1999 suggested a doughnut shape, which has never been observed in the leucinerich repeat (LRR) superfamily, to which ALS belongs. In this study, we built a new ALS structural model, analysed its glycosylation and charge distribution and studied mechanisms by which missense mutations affect protein structure. We used three structure prediction servers and integrated their results with information derived from ALS experimental studies. The ALS model was built at high confidence using Toll-like receptor protein templates and resembled a horseshoe with an extensively negatively charged concave surface. Enrichment in prolines and disulphide bonds was found at the ALS N-and C-termini. Moreover, seven N-glycosylation sites were identified and mapped. ALS mutations were predicted to affect protein structure by causing loss of hydrophobic interactions (p.Leu134Gln), alteration of the amino acid backbone (p.Leu241Pro, p.Leu172Phe and p.Leu244Phe), loss of disulphide bridges (p.Cys60Ser and p.Cys540Arg), change in structural constrains (p.Pro73Leu), creation of novel glycosylation sites (p.Asp440Asn) or alteration of LRRs (p.Asn276Ser). In conclusion, our ALS structural model was identified as a highly confident prediction by three independent methods and disagrees with the previously published ALS model. The new model allowed us to analyse the ALS core and its caps and to interpret the potential structural effects of ALS mutations.

A Novel Homozygous Mutation of the <b><i>IGFALS</i></b> Gene in a Female Adolescent: Indirect Evidence for a Contributing Role of the Circulating IGF-I Pool in the Pubertal Growth Spurt

“…Most of these mutations are in the leucine-rich repeats (LRR), where they are predicted to disrupt the conformation of the protein, thereby rendering ALS unable to bind to the binary complex formed by IGF-I with IGFBP-3 [24]. Indeed, the LRR of ALS is able to participate in protein-protein interaction as described for other LRR proteins.…”

“…The most likely outcome of the frameshift point mutation reported in our patient will be non-sense-mediated decay of the truncated mRNA, and hence absence of the protein, fitting with the biochemical result. Even if the protein were translated, it would lack the LRR between amino acids 574 and 618 required for protein-protein interactions, and would almost undoubtedly result in a nonfunctional protein [24]. …”

Analysis of acid-labile subunit and its usefulness in pediatrics

“…3 The first structural model was described in 1999. More recently, David et al 2 have introduced a new model analyzing ALS glycosylation, charge distribution, and the mechanisms by which missense mutations affected the protein structure. 2 This structural model contains 6 disulphide bridges, which are mainly clustered towards the N-terminus (LRR-1) and the C-terminus .…”

“…In such complex, ALS is associated with some of the proteins of the insulinlike growth factor (IGF) family and one of the insulin-like growth factorbinding proteins (IGFBP): IGFBP3 or IGFBP5. 2,3 The IGF family is comprised of 2 peptide hormones synthesized by the liver: IGF1 and IGF2. These share approximately 50% of their amino acid sequence and a large portion of their sequence displays homology to proinsulin but, unlike the latter, they retain C-peptide and are therefore longer.…”

Clinical Features of a New Acid-Labile Subunit <b><i>(IGFALS)</i></b> Heterozygous Mutation: Anthropometric and Biochemical Characterization and Response to Growth Hormone Administration

“…The ALS model, initially described as a doughnut-shaped structure [4], resembled a horseshoe arrangement characterized by an extensively negatively charged concave surface, six S-S bridges and seven potential N-glycosylation sites. According to this structural model, most of the previously described inactivating mutations could cause a distortion of the ALS protein structure [5]. …”