An intended secreted protein is barely detectable in culture medium. Increasing production scale is tempting, but it may multiply the same unresolved problem. Before changing scale, separate three questions: was the protein expressed, did it reach the medium, and can the chosen assay detect it in that sample?
Examining both cellular lysate and medium is a useful first distinction. Detection in cells but not medium supports a different investigation from failure to detect either fraction. Thermo Fisher recommends checking both fractions when troubleshooting secretory expression and notes that a secretion signal is not guaranteed to work for every protein. Treat the signal sequence as a feature to evaluate, not a universal solution.
Map the antibody epitope or detection tag against the intended mature protein. Processing can change which sequence is present in the final product. A signal detected through one terminal tag should not be assumed to report every processed form. If the result is unexpected, an independent detection approach can help distinguish low abundance from failure of the original assay.
Equal volumes of lysate and medium do not necessarily represent equal fractions of the culture. Record sample volumes, concentration steps, and cell numbers so that the comparison is interpretable. Include an assay-appropriate reference to establish that the sample matrix and detection system can reveal the target. A negative band without a useful positive reference leaves several explanations open.
A small pilot can compare the existing construct with a justified design alternative while keeping the rest of the experiment consistent. If external mammalian protein expression support is used, share the evidence from both fractions and specify whether the immediate objective is expression, secretion, or recovery of a detectable final product. Those are related but distinct project goals.
Intracellular detection does not, on its own, identify the exact trafficking defect. Likewise, a low medium concentration does not prove that synthesis failed. Describe the observation first, then choose the next experiment to distinguish plausible explanations. That approach is usually more informative than changing host, signal sequence, and assay simultaneously.
Thermo Fisher Scientific. Basic Mammalian Expression Support—Troubleshooting.
Thermo Fisher Scientific. pSecTag2 A, B, & C Mammalian Expression Vectors: secretion-signal and fusion-protein architecture.