Question

A researcher uses molecular biology techniques to insert a human lysosomal membrane protein into bacterial cells to produce large quantities of this protein for later study. However, only small quantities of this protein result in these cells. What is a possible explanation for this result? (A) The membrane protein requires processing in the ER and Golgi, which are missing in the bacterial cells. (B) Bacteria do not make membrane proteins. (C) Bacteria do not use different transcription factors than humans, so the gene was not expressed. (D) Bacteria do not have enough tRNAs to make this protein sequence.

Short answer

The most plausible explanation for the low yield of human lysosomal membrane protein in bacterial cells is: \[ \textrm{(A) The membrane protein requires processing in the ER and Golgi, which are} \newline \textrm{missing in the bacterial cells.} \]

Step-by-step solution

Option A: Processing in ER and Golgi

Bacterial cells lack the endoplasmic reticulum (ER) and Golgi apparatus, which are essential for the proper processing and post-translational modification of eukaryotic membrane proteins. These organelles play crucial roles in the proper folding, trafficking, and integration of membrane proteins into membranes. The absence of the ER and Golgi in bacteria may lead to insufficient processing and folding, resulting in low protein production.

Option B: Bacteria not making membrane proteins

This statement is incorrect. Bacteria do produce membrane proteins, although they might be different in structure and function from human membrane proteins. Bacterial membrane proteins play important roles in various cellular processes such as transport, signaling, and metabolism.

Option C: Different transcription factors

This statement is also incorrect. While bacteria and humans indeed use different transcription factors, this would not affect the expression of the protein itself. When inserting a foreign gene, such as a human gene, into bacteria for protein production, researchers often use a bacterial promoter and other regulatory elements that function with bacterial transcription factors. This allows the gene to be expressed by bacterial machinery.

Option D: Insufficient tRNAs

This statement is unlikely. While bacteria and humans may have slightly different tRNA pools, it is generally possible for bacterial translation systems to synthesize a wide range of protein sequences, including human proteins. Researchers usually optimize the codon usage in the gene to enhance protein expression in bacterial systems. Therefore, based on our analysis, the most plausible explanation for the low yield of human lysosomal membrane protein in the bacterial cells can be found in option A: The membrane protein requires processing in the ER and Golgi, which are missing in the bacterial cells.

Key concepts

Post-Translational Modifications

After proteins are synthesized, they often undergo further modifications to become fully functional. These changes, known as post-translational modifications (PTMs), are essential for proper protein structure and function. Examples include phosphorylation, glycosylation, and lipidation. In eukaryotic cells, many PTMs occur in the endoplasmic reticulum (ER) and Golgi apparatus. However, bacteria lack these organelles, leading to difficulties when expressing eukaryotic proteins, such as the human lysosomal membrane protein mentioned in the exercise. As a result, these proteins may be produced in lower quantities or may not fold correctly without these modifications, leading to degradation or malfunction.

In the context of our exercise, PTMs are pivotal since the absence of the appropriate modification machinery in bacteria can lead to a misunderstanding about the efficiency of bacterial protein expression systems and result in poor yield of the desired protein.

Endoplasmic Reticulum and Golgi Apparatus

The endoplasmic reticulum (ER) and Golgi apparatus are crucial compartments within eukaryotic cells where proteins are modified, folded, and transported. The ER assists in protein folding and PTMs, while the Golgi apparatus further modifies proteins and sorts them for transport to their final destinations.

For proteins like human lysosomal membrane proteins, which depend on this sophisticated eukaryotic machinery, bacterial cells don't offer the required cellular environment. The solution in the exercise points to this very issue: the desired protein's complex eukaryotic processing needs cannot be met by bacterial facilities, hence, yielding only small amounts of the functional protein in bacterial systems.

Bacterial Protein Synthesis

When discussing bacterial protein synthesis, it's important to note that bacteria, as prokaryotes, have a simpler cellular structure than eukaryotes. Their process of protein synthesis occurs in the cytoplasm and is fundamentally similar to eukaryotic protein synthesis, though with simpler machinery involved.

Bacteria are capable of producing a wide array of proteins, including membrane proteins. However, when it comes to expressing eukaryotic proteins, bacteria may struggle due to the lack of specific organelles and PTM processes. This is critical for students to understand, as it clarifies why in our exercise scenario, the bacteria produced insufficient human lysosomal membrane protein.

Transcription Factors in Gene Expression

Transcription factors are proteins that bind to specific DNA sequences, controlling the flow (or transcription) of genetic information from DNA to mRNA. They play a pivotal role in gene expression. In the exercise, it's mentioned that bacteria and humans use different transcription factors. However, researchers can introduce genes into bacterial cells with bacterial promoters and regulatory elements compatible with bacterial transcription factors.

This compatibility allows the bacteria to express the introduced human protein. Students should realize that although bacteria utilize different transcription machinery compared to humans, genetic engineering allows us to circumvent this issue to a significant extent, but this isn't the limiting factor in the case of our exercise.

Codon Optimization for Protein Expression

In genetic engineering, codon optimization is often performed to enhance protein expression in an organism that is not the original source of the DNA. Codons, which are sequences of three nucleotides in the mRNA, encode for specific amino acids. Since different organisms prefer different codons for the same amino acid, optimizing a gene's codon usage for the host organism can significantly increase protein production.

Optimizing codon usage involves modifying the DNA sequence of a gene without altering the amino acid sequence of the protein it encodes. In the context of our exercise, despite the bacteria's capability of translating human proteins, the researcher needs to ensure that the human gene's codons are optimized for the bacterial system to maximize protein yield.