My first authorship is being turned into co-first authorship, what can I do? I am currently submitting a paper for Nature Structural &amp; Molecular Biology. I worked on this project for the last four years and was the only one working on this project. I had everything going very well. In order to get the crystallization, I gave the plasmid construct to another laboratory which has expertise in crystallization. Fortunately, they got the crystals.
After the crystallization got successful, one researcher (say, A) from their lab came to my lab and my professor told me to teach him all the methods. I happily agreed to do so. After A went back to his lab, I got the draft of the paper from my professor, where, my name appeared as first co-author! I almost fainted. There were 20 figures in the paper. 13 were contributed by me, 4 by A and rest by 2 other people. The paper was mostly about my work. But in the contribution section, my prof and the prof of A  mentioned that I and A had equal contribution. In fact it is mentioned that A was solely contributor of crystals and he equally contributed in all other results.
I discussed this issue with my prof, but he tells that he is getting pressure from A's prof to make A the first author. And my prof does not want to estrange relationship with A's prof. I am heart-broken and I don't know what to do.
Please suggest me what to do.
Edit 2021: It has been quite some time since I posted this question. I accepted the authorship as recommended by my advisor. Now I feel mature and realize that maintaining a cordial relationship with one's advisor and colleagues is more important than anything else. If one is capable enough, s/he will get more opportunities to prove their worth.
First, try to calm down and don't react hastily.

Your situation is very frustrating, but it is not horrible. Let's say that the current author ordering would be the final one. You would be what's known as a co-first author. While it would be slightly more advantageous to be the sole first author, in my experience being co-first practically will not have a significant negative effect on your CV (BTW - are you a grad student or postdoc?). This is important for you to realize - the worst-case scenario that you mention is upsetting, but definitely not a disaster.

Some additional points:


You can try explaining your perspective to your professor. If you originally decided that you would be the sole first author, you should definitely remind him/her of that. Do this politely, of course, but show your professor this is a big deal for you. This could cause the professor to reconsider, depending on his/her personality.
Instead of being the sole first author, you can consider being co-first but listed first. This doesn't make a big difference, but people might associate you with the paper more easily (Whatshername et al.).
Consider that regardless of the number of figures, the other author might deserve being co-first. For example, in many papers an experimentalist produces the data and a computational person analyzes the data and produces most of the figures - and they would often be equal co-first authors (even though each of them could think of themselves doing "most of the work"). So I do not know all the details of who did what, but it does not sound completely implausible that the other person did work which justifies being co-first.
Regardless of what happens, I suggest you talk with your professor about how to get a more satisfactory outcome next time (if this is relevant for you). Discuss authorship in advance and what happens when things change.


Good luck!