A karyophilic protein forms a stable complex with cytoplasmic components prior to nuclear pore binding

Area of Science:

• Cell biology
• Molecular biology
• Biochemistry

Background:

• Nuclear transport is crucial for eukaryotic cell function.
• Karyophilic proteins require cytoplasmic factors for nuclear pore targeting.
• The nuclear location signal (NLS) is key for nuclear import.

Purpose of the Study:

• To identify and characterize cytoplasmic factors involved in karyophilic protein targeting to nuclear pores.
• To elucidate the composition and function of the nuclear pore-targeting complex.

Main Methods:

• Digitonin-permeabilized cell-free transport assay.
• Biochemical fractionation of cytoplasmic extracts.
• Complex formation and binding assays.
• Reconstitution experiments.

Main Results:

• A cytoplasmic fraction containing specific binding factors was isolated.
• These factors form a stable complex with karyophilic proteins via the NLS.
• This complex, termed the nuclear pore-targeting complex (NTC), mediates nuclear pore binding.
• The NTC consists of the karyophilic protein and four associated proteins (54, 56, 66, and 90 kDa).
• A subcomplex of 54 and 90 kDa proteins is sufficient for targeting karyophiles to nuclear pores.

Conclusions:

• The nuclear pore-targeting complex is a key mediator of nuclear protein import.
• The NTC facilitates the initial step of nuclear transport by binding to the nuclear pore.
• Specific protein components, particularly the 54 and 90 kDa proteins, are essential for NTC function.