Messenger RNA (mRNA) exits the nucleus through nuclear pores, which are large protein complexes that span the nuclear envelope, the double membrane that surrounds the nucleus. The process of mRNA export is facilitated by a variety of proteins, including:

1. mRNA export factors: These proteins bind to the mRNA molecule and help it to form a complex that can be transported through the nuclear pore.

2. RanGTP: This small GTPase (guanosine triphosphatase) protein is essential for the export of mRNA. RanGTP is located in the cytoplasm and binds to the mRNA export factors, causing them to undergo a conformational change that allows them to interact with the nuclear pore complex.

3. Nuclear pore complex proteins: These proteins form a channel through the nuclear envelope and allow the mRNA-export factor complex to pass through.

Once the mRNA-export factor complex has passed through the nuclear pore, it is released into the cytoplasm, where it can be translated into protein.

Several important steps are involved in the process of mRNA export:

1. Intron splicing: Before mRNA can be exported from the nucleus, it must undergo intron splicing, a process that removes non-coding introns from the pre-mRNA molecule. This process is carried out by a large protein complex called the spliceosome.

2. mRNA capping: The 5' end of the mRNA molecule is capped with a modified guanine nucleotide called 7-methylguanosine (m7G). This cap protects the mRNA from degradation and helps it to bind to the ribosome for translation.

3. mRNA polyadenylation: The 3' end of the mRNA molecule is polyadenylated, a process that adds a tail of adenine nucleotides to the mRNA. This tail also protects the mRNA from degradation and helps it to bind to proteins that are involved in its transport and translation.

4. mRNA nuclear export: Once the mRNA molecule has been capped and polyadenylated, it can be exported from the nucleus through the nuclear pore complex. This process is mediated by a family of proteins called exportins. These proteins bind to the mRNA molecule and guide it through the nuclear pore complex.

The mRNA export process is essential for the proper regulation of gene expression. By controlling the export of mRNA from the nucleus, cells can control the amount of protein that is produced from a given gene.