Neurofilaments are obligate heteropolymers in vivo.
Neurofilaments (NFs), composed of three distinct subunits NF-L, NF-M, and NF-H, are neuron-specific intermediate filaments present in most mature neurons. Using DNA transfection and mice expressing NF transgenes, we find that despite the ability of NF-L alone to assemble into short filaments in vitro NF-L cannot form filament arrays in vivo after expression either in cultured cells or in transgenic oligodendrocytes that otherwise do not contain a cytoplasmic intermediate filament (IF) array. Instead, NF-L aggregates into punctate or sheet like structures. Similar nonfilamentous structures are also formed when NF-M or NF-H is expressed alone. The competence of NF-L to assemble into filaments is fully restored by coexpression of NF-M or NF-H to a level approximately 10% of that of NF-L. Deletion of the head or tail domain of NF-M or substitution of the NF-H tail onto an NF-L subunit reveals that restoration of in vivo NF-L assembly competence requires an interaction provided by the NF-M or NF-H head domains. We conclude that, contrary to the expectation drawn from earlier in vitro assembly studies, NF-L is not sufficient to assemble an extended filament network in an in vivo context and that neurofilaments are obligate heteropolymers requiring NF-L and NF-M or NF-H.
Lee
M K
MK
Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Xu
Z
Z
Wong
P C
PC
Cleveland
D W
DW
D000595:Amino Acid Sequence; D000818:Animals; D002478:Cells, Cultured; D005347:Fibroblasts / Q000737:chemistry / Q000378:metabolism; D007382:Intermediate Filaments / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure; D051379:Mice; D008822:Mice, Transgenic; D008969:Molecular Sequence Data; D016900:Neurofilament Proteins / Q000737:chemistry / Q000378:metabolism; D009836:Oligodendroglia / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure*; D011108:Polymers; D014162:Transfection
D016900:Neurofilament Proteins; D011108:Polymers; C044882:neurofilament protein L; C061195:neurofilament protein M