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Table 1 Isoforms of major components of the IGF1-Akt pathway

From: Regulation of skeletal muscle growth by the IGF1-Akt/PKB pathway: insights from genetic models

Component Isoforms References
IGF11 Two (mouse: 1A, 1B) or three (human: 1A, 1B & 1C) isoforms, differing in the C terminal peptide (E peptide); another two (mouse: 2A, 2B) or three (human: 2A, 2B & 2C) isoforms differing in signal peptide because of utilization of exon 2 instead of exon 1 [49]
IGF1 receptor Heterotetramer made of two α-subunit (IGF-binding) and two β-subunit (tyrosine kinase). No isoforms, but hybrids with insulin receptor are present in skeletal muscle [112]
IRS2 IRS1 and IRS2 [65, 113]
PDK13 No isoforms [114]
PI3K4 (class I) Heterodimer of p85 regulatory and p110 catalytic subunits. p85: three isoforms (p85α, p55α, p50α) encoded by a single gene; two other isoforms (p85β, p55γ) coded by other genes. p55γ is not expressed in muscle. p110: three isoforms (p110α, p110β, p110δ). p110δ is not expressed in muscle [115]
Akt/PKB5 Akt1/PKBα, Akt2/PKBβ & Akt3/PKBγ. Akt3 is not expressed in muscle [4, 116]
mTOR6 No isoforms, but mTOR can interact with different partners: Raptor in the rapamycin-sensitive complex mTORC1, or Rictor in the rapamycin-insensitive complex mTORC2 [5]
S6K7 S6K1 and S6K2 [117119]
4EBP8 4EBP1, 4EBP2, 4EBP3. 4EBP3 is not expressed in muscle [120]
FoxO 4 isoforms: FoxO1, FoxO3, FoxO4 and FoxO6. FoxO6 is not expressed in muscle [121]
  1. 1Insulin-like growth factor.
  2. 2Insulin receptor substrate.
  3. 33-Phosphoinositide-dependent kinase 1, coded by the PDPK1 gene (unfortunately the same abbreviation PDK1 is also used to indicate the pyruvate dehydrogenase kinase isoform 1, coded by the PDK1 gene).
  4. 4Phosphoinositide-3 kinase.
  5. 5Protein kinase B.
  6. 6Mammalian target of rapamycin.
  7. 7S6 kinase.
  8. 8Eukaryotic initiation factor 4E binding protein.