Kelch proteins: emerging roles in skeletal muscle development and diseases

Abstract

Our understanding of genes that cause skeletal muscle disease has increased tremendously over the past three decades. Advances in approaches to genetics and genomics have aided in the identification of new pathogenic mechanisms in rare genetic disorders and have opened up new avenues for therapeutic interventions by identification of new molecular pathways in muscle disease. Recent studies have identified mutations of several Kelch proteins in skeletal muscle disorders. The Kelch superfamily is one of the largest evolutionary conserved gene families. The 66 known family members all possess a Kelch-repeat containing domain and are implicated in diverse biological functions. In skeletal muscle development, several Kelch family members regulate the processes of proliferation and/or differentiation resulting in normal functioning of mature muscles. Importantly, many Kelch proteins function as substrate-specific adaptors for Cullin E3 ubiquitin ligase (Cul3), a core component of the ubiquitin-proteasome system to regulate the protein turnover. This review discusses the emerging roles of Kelch proteins in skeletal muscle function and disease.

Keywords: BACK; BTB; Congenital myopathy; Cul3; Differentiation; Dystrophy; Kelch; Nemaline myopathy; Proliferation; Proteasome; Skeletal muscle; Ubiquitination.

Figure 1

The Kelch Superfamily. (A) The Kelch family consists of 63 proteins that are subclassified in to KLHL, KBTBD and KLHDC subfamilies. (B) Structure of Kelch domain of rat KLHL41 (PDB code 2WOZ) comprising six repeats that form the complete Kelch domain. The structure was generated using PyMOL (http://www.pymol.org). (C) Prototype members of different subfamilies showing different domain organization. KLHL proteins have an N-terminal BTB/POZ, a BACK and C-terminal Kelch repeats. KBTBD proteins contain an N-terminal BTB domain and Kelch repeats. The BACK domain is normally absent in KBTBD proteins. KLHDC proteins lack both BTB/POZ and BACK domains and contain either Kelch repeats alone or with other domains such as transmembrane (for example, KLHDC7A), Glycine rich (for example, KLHDC10), or Lish and CTLH domains (for example, MKLN1).

Figure 2

Phylogenetic analysis showing relationships between human Kelch protein family members. (A) Phylogenetic tree of full-length amino acid sequences of human proteins were aligned. (B) Phylogenetic tree of amino acid sequences of Kelch domains. Phylogenetic trees were constructed by maximum-likelihood method using BLOSUM matrix in MEGA 6.06. Reference sequences used for alignments are indicated at right of each protein name. Blue highlighting indicates KBTBD subfamily members; green indicates KLHDC subfamily members. *, proteins involved in neuromuscular diseases; **, family members implicated in cancer; #, proteins whose defects cause other inherited diseases (Table  2). Scale bars indicate relative distances and represent the degree of differences between the sequences.

Figure 3

Kelch proteins act as a substrate specific adaptors for E3-ubiquitinin protein complex. (A) Cullin3 complex is Nedd8 (N8) modified and recruits E2-bound ubiquitin through RING-finger protein Rbx1. The assembly of a functional ubiquitination complex requires the binding of Cul3-E2 complex to substrate specific Kelch adaptor proteins. Cul3 directly binds to N-terminal BTB domain of Kelch protein and this E3-ubiquitination complex interacts with substrates (for example, S1, S2, S3) by C-terminal Kelch-repeat containing domains of Kelch proteins, causing ubiquitination of the target proteins and subsequent degradation (or stabilization and so on) by the proteasome system. This results in normal protein turnover of proteins required for normal functioning of muscle resulting in healthy skeletal muscles. (B) The deficiency of Kelch proteins (such as disease causing KLHL9, KLHL40, KLHL41, and KBTBD13) prevents the assembly of functional Cullin3 ubiquitination complex thereby perturbing the protein turnover process. In the model shown here, this results in accumulation of abnormal proteins (for example, S1, S2, S3) leading to unavailability of normal proteins in skeletal muscle leading to a diseased state.