You can see firefly luciferase+tag (for example N-Terminal His-tag )

Ususally firefly luciferase is a 62 kDa protein

"luciferase is a dimer formed from the luxA and luxB genes, while the remainder of the genes (luxCDE) are responsible for protein products that catalyze production and turnover of the required aldehyde substrate "

Article Article In Vivo Bioluminescent Imaging (BLI): Noninvasive Visualizat...

Thee are many reasons why you seen on a native gel not the expected molecular weight. To find and answer to your problem, I have a few questions:

1. Is your protein of interest physiologically a monomer or a dimer? If yes, that is what you may be seeing.

2. If not, how stresful is the isolation of your protein of interest? Some proteins have the tendency to dimereize or even form bigger aggregates in oxidizing conditions.

3. What detergent did you use?

4. Have you varied he concentration of the detergent?

I have attached 2 papers that I find really helpful.

Good luck!

what is the MW of the luciferase dimer? is it alpha+beta or alpha+betax2?

First get a proper markers ladder, so you know the MW of your bands. BN can be tricky sometimes.

NB: Why do you think it is a problem that your target protein is a dimer? Maybe this is how it is in situ. But if you definitely want to break the dimer - increase a detergent/protein ratio.

It's not like an SDS PAGE, which depends on MW only. Here, in this case, depends on many factors; MW, Charge, & Size.